Fix a possible race in mt7921_pm_power_save_work() if rx/tx napi
schedules ps_work and we are currently accessing device register
on a different cpu.

Fixes: 1d8efc74 ("mt76: mt7921: introduce Runtime PM support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>