wifi: mt76: mt7921: add mt7921_mutex_acquire at mt7921_sta_set_decap_offload

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: 24299fc8

 ("mt76: mt7921: enable rx header traslation offload")
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>