mt76: mt7921s: fix firmware download random fail

To avoid racing problems in chip, mt7921s should reacquire drv-own after
firmware semaphore is released.

Fixes: 78b21758

 ("mt76: mt7921s: fix bus hang with wrong privilege")
Signed-off-by: YN Chen <yn.chen@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>