wifi: mt76: mt7921: add mt7921_mutex_acquire at mt7921_[start, stop]_ap

Add mt7921_mutex_acquire at mt7921_[start, stop]_ap to fix the race
with the context holding dev->muxtex and the driver might access the
device in low power state.

Fixes: 9d958b60

 ("mt76: mt7921: fix command timeout in AP stop period")
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>