mt76: mt7615: fix a possible race enabling/disabling runtime-pm

Similar to mt7921 driver, fix a possible race enabling/disabling
runtime-pm between mt7615_pm_set() and mt7615_poll_rx().
mt7615_pm_wake_work() always schedules rx-napi callback and it will
trigger mt7615_pm_power_save_work routine putting the chip in low-power
state even if we are disabling runtime-pm deferring the actual chip wake
at the next access.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>