Fix mixing for up to 6 steppers (#15374)

Marlin/src/feature/mixing.h

     static inline void copy_mix_to_color(mixer_comp_t (&tcolor)[MIXING_STEPPERS]) {
       // Scale each component to the largest one in terms of COLOR_A_MASK
       // So the largest component will be COLOR_A_MASK and the other will be in proportion to it
-      const float scale = (COLOR_A_MASK) * RECIPROCAL(float(_MAX(mix[0], mix[1])));
+      const float scale = (COLOR_A_MASK) * RECIPROCAL(_MAX(
+        LIST_N(MIXING_STEPPERS, mix[0], mix[1], mix[2], mix[3], mix[4], mix[5])
+      ));
 
       // Scale all values so their maximum is COLOR_A_MASK
       MIXER_STEPPER_LOOP(i) tcolor[i] = mix[i] * scale;
 
       #ifdef MIXER_NORMALIZER_DEBUG
-        SERIAL_ECHOLNPAIR("Mix [", int(mix[0]), ", ", int(mix[1]), "] to Color [", int(tcolor[0]), ", ", int(tcolor[1]), "]");
+        SERIAL_ECHOPGM("Mix [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(mix[0]), int(mix[1]), int(mix[2]), int(mix[3]), int(mix[4]), int(mix[5]));
+        SERIAL_ECHOPGM(" ] to Color [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(tcolor[0]), int(tcolor[1]), int(tcolor[2]), int(tcolor[3]), int(tcolor[4]), int(tcolor[5]));
+        SERIAL_ECHOLNPGM(" ]");
       #endif
     }
 
       float ctot = 0;
       MIXER_STEPPER_LOOP(i) ctot += color[j][i];
       //MIXER_STEPPER_LOOP(i) mix[i] = 100.0f * color[j][i] / ctot;
-      mix[0] = mixer_perc_t(100.0f * color[j][0] / ctot);
-      mix[1] = 100 - mix[0];
+      MIXER_STEPPER_LOOP(i) mix[i] = mixer_perc_t(100.0f * color[j][i] / ctot);
+
       #ifdef MIXER_NORMALIZER_DEBUG
-        SERIAL_ECHOLNPAIR("V-tool ", int(j), " [", int(color[j][0]), ", ", int(color[j][1]), "] to Mix [", int(mix[0]), ", ", int(mix[1]), "]");
+        SERIAL_ECHOPAIR("V-tool ", int(j), " [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(color[j][0]), int(color[j][1]), int(color[j][2]), int(color[j][3]), int(color[j][4]), int(color[j][5]));
+        SERIAL_ECHOPGM(" ] to Mix [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(mix[0]), int(mix[1]), int(mix[2]), int(mix[3]), int(mix[4]), int(mix[5]));
+        SERIAL_ECHOLNPGM(" ]");
       #endif
     }
 
     static inline void update_mix_from_gradient() {
       float ctot = 0;
       MIXER_STEPPER_LOOP(i) ctot += gradient.color[i];
-      mix[0] = (mixer_perc_t)CEIL(100.0f * gradient.color[0] / ctot);
-      mix[1] = 100 - mix[0];
+      MIXER_STEPPER_LOOP(i) mix[i] = (mixer_perc_t)CEIL(100.0f * gradient.color[i] / ctot);
+
       #ifdef MIXER_NORMALIZER_DEBUG
-        SERIAL_ECHOLNPAIR("Gradient [", int(gradient.color[0]), ", ", int(gradient.color[1]), "] to Mix [", int(mix[0]), ", ", int(mix[1]), "]");
+        SERIAL_ECHOPGM("Gradient [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(gradient.color[0]), int(gradient.color[1]), int(gradient.color[2]), int(gradient.color[3]), int(gradient.color[4]), int(gradient.color[5]));
+        SERIAL_ECHOPGM(" ] to Mix [ ");
+        SERIAL_ECHOLIST_N(MIXING_STEPPERS, int(mix[0]), int(mix[1]), int(mix[2]), int(mix[3]), int(mix[4]), int(mix[5]));
+        SERIAL_ECHOLNPGM(" ]");
       #endif
     }