mt76: mt7615: fix a possible NULL pointer dereference in mt7615_pm_wake_work

Initialize wcid to global_wcid if msta is NULL in mt7615_pm_wake_work
routine since wcid will be dereferenced running mt76_tx()

Fixes: 2b8cdfb2 ("mt76: mt7615: wake device before pushing frames in mt7615_tx")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>