autotemp_factor is not value_celsius_diff

See https://github.com/MarlinFirmware/Marlin/issues/9852#issuecomment-369861822

Marlin/src/module/planner.cpp

 #if ENABLED(AUTOTEMP)
 
   void Planner::autotemp_M104_M109() {
-    autotemp_enabled = parser.seen('F');
-    if (autotemp_enabled) autotemp_factor = parser.value_celsius_diff();
+    if ((autotemp_enabled = parser.seen('F'))) autotemp_factor = parser.value_float();
     if (parser.seen('S')) autotemp_min = parser.value_celsius();
     if (parser.seen('B')) autotemp_max = parser.value_celsius();
   }