wifi: mt76: mt7921: drop ieee80211_[start, stop]_queues in driver

The firmware would be in charge of braking and continuing the traffic while
the channel contexts are switching between different BSS and HW SCAN in the
background.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>