fix M81 issue

[ellensp]

Description

M81 has been reported to no longer work.
A fix was also provided
This PR adds the fix.

This was in the code prior to 22304

Benefits

M81 works again

Related Issues

#23396 (comment)
#22304

[thinkyhead]

Did you confirm this yourself?

This fix is strange. Marlin already does this in setup():

  #if ENABLED(PSU_CONTROL)
    SETUP_RUN(powerManager.init());
  #endif

…which calls…

void Power::init() {
  psu_on = ENABLED(PSU_DEFAULT_OFF);              // Set opposite state to get full power_off/on
  TERN(PSU_DEFAULT_OFF, power_off(), power_on());
}

Therefore:

• If you have PSU_DEFAULT_OFF enabled it causes psu_on to be initialized as true and then power_off() is called which turns off the power and sets it false.
• If you have PSU_DEFAULT_OFF disabled it causes psu_on to be initialized as false and then power_on() is called which turns on the power and sets it true.

If you don't enable PSU_CONTROL at all then various things are turned off by M81, but there is no psu_on flag and control of the PSU is not available.

[ellensp]

Tested on ramps, works without patch