Before preparing the new beacon, check the queue status, flush out all
previous beacons and buffered multicast packets, then (if necessary)
try to recover more gracefully from a stuck beacon condition by making a
less invasive attempt at getting the MAC un-stuck.

Fixes: c8846e10 ("mt76: add driver for MT7603E and MT7628/7688")
Signed-off-by: Felix Fietkau <nbd@nbd.name>