Move LA reset from G28 to G80, do not reset on 0

In preparation for #2161, use MBL (G80) as a "new print" boundary
instead of just re-homing to ensure the reset is issued only once for
each print.

Similarly, do not reset the autodetection when LA is disabled via M900
K0. This can/will be used during a print if different quality settings
are used for different filling roles.

Firmware/Marlin_main.cpp

@@ -2078,7 +2078,6 @@ inline void gcode_M900() {
         SERIAL_ECHOLNPGM("K out of allowed range!");
 #else
     if (newK == 0) {
-        la10c_reset();
         extruder_advance_K = 0;
     }
     else if(newK > 0)
@@ -4148,9 +4147,6 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
     // --------------------------------------------
     case 28: 
     {
-#ifndef LA_NOCOMPAT
-      la10c_reset();
-#endif
       long home_x_value = 0;
       long home_y_value = 0;
       long home_z_value = 0;
@@ -4674,6 +4670,11 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
 	case_G80:
 	{
 		mesh_bed_leveling_flag = true;
+#ifndef LA_NOCOMPAT
+        // When printing via USB there's no clear boundary between prints. Abuse MBL to indicate
+        // the beginning of a new print, allowing a new autodetected setting just after G80.
+        la10c_reset();
+#endif
 #ifndef PINDA_THERMISTOR
         static bool run = false; // thermistor-less PINDA temperature compensation is running
 #endif // ndef PINDA_THERMISTOR