Proportional Autotemp followup (#17585)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/module/planner.cpp

       const int16_t target = thermalManager.degTargetHotend(active_extruder);
       autotemp_min = target + AUTOTEMP_MIN_P;
       autotemp_max = target + AUTOTEMP_MAX_P;
-      autotemp_factor = AUTOTEMP_FACTOR_P;
     #endif
 
     if (parser.seenval('S')) autotemp_min = parser.value_celsius();
     if (parser.seenval('B')) autotemp_max = parser.value_celsius();
-    if (parser.seenval('F')) autotemp_factor = parser.value_float();
-    if (!autotemp_factor) autotemp_enabled = false; // F0 will disable autotemp
-  }
 
+    // When AUTOTEMP_PROPORTIONAL is enabled, F0 disables autotemp.
+    // Normally, leaving off F also disables autotemp.
+    autotemp_factor = parser.seen('F') ? parser.value_float() : TERN(AUTOTEMP_PROPORTIONAL, AUTOTEMP_FACTOR_P, 0);
+    autotemp_enabled = autotemp_factor != 0;
+  }
 #endif