mt76: mt7615: wake device in mt7615_update_channel before access regmap

Introduce mt7615_update_survey utility routine in order to compute
survey stats without waking up the device since it runs holding mt76 lock.
Run mt7615_pm_wake directly in mt7615_update_channel since it can run
with mt76.mutex held if called by mac80211

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