wifi: mt76: mt7615: fix possible race in mt7615_mac_sta_poll

wifi: mt76: mt7615: fix possible race in mt7615_mac_sta_poll

Grab sta_poll_lock spinlock in mt7615_mac_sta_poll routine in order to
avoid possible races with mt7615_mac_add_txs() or mt7615_mac_fill_rx()
removing msta pointer from sta_poll_list.

Fixes: a621372a04ac ("mt76: mt7615: rework mt7615_mac_sta_poll for usb code")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/48b23404b759de4f1db2ef85975c72a4aeb1097c.1684938695.git.lorenzo@kernel.org