wifi: mt76: mt7921: reset msta->airtime_ac while clearing up hw value

We should reset mstat->airtime_ac along with clear up the entries in the
hardware WLAN table for the Rx and Rx accumulative airtime. Otherwsie, the
value msta->airtime_ac - [tx, rx]_last may be a negative and that is not
the actual airtime the device took in the last run.

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