Introduce a mutex in order to avoid a race between mt7615_mcu_lp_drv_pmctrl and
mt7615_mcu_fw_pmctrl routines since they are run two independent works

Fixes: 1f549009 ("mt76: mt7615: do not request {driver,fw}_own if already granted")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>