⚡️ Exit from ISR on AVR already does sei

Co-Authored-By: tombrazier <68918209+tombrazier@users.noreply.github.com>

Marlin/src/module/stepper.cpp

@@ -1600,8 +1600,11 @@ void Stepper::isr() {
   // Set the next ISR to fire at the proper time
   HAL_timer_set_compare(MF_TIMER_STEP, hal_timer_t(next_isr_ticks));
 
-  // Don't forget to finally reenable interrupts
+  // Don't forget to finally reenable interrupts on non-AVR.
+  // AVR automatically calls sei() for us on Return-from-Interrupt.
+  #ifndef __AVR__
     hal.isr_on();
+  #endif
 }
 
 #if MINIMUM_STEPPER_PULSE || MAXIMUM_STEPPER_RATE