Fix IS_PROBE_PIN macro (#19024)

Marlin/src/inc/Conditionals_post.h

@@ -1622,7 +1622,7 @@
 //
 
 // Is an endstop plug used for extra Z endstops or the probe?
-#define IS_PROBE_PIN(A,M) (HAS_CUSTOM_PROBE_PIN && Z_MIN_PROBE_PIN == P)
+#define IS_PROBE_PIN(A,M) (HAS_CUSTOM_PROBE_PIN && Z_MIN_PROBE_PIN == A##_##M##_PIN)
 #define IS_X2_ENDSTOP(A,M) (ENABLED(X_DUAL_ENDSTOPS) && X2_USE_ENDSTOP == _##A##M##_)
 #define IS_Y2_ENDSTOP(A,M) (ENABLED(Y_DUAL_ENDSTOPS) && Y2_USE_ENDSTOP == _##A##M##_)
 #define IS_Z2_ENDSTOP(A,M) (ENABLED(Z_MULTI_ENDSTOPS) && Z2_USE_ENDSTOP == _##A##M##_)