wifi: mt76: sdio: fix the deadlock caused by sdio->stat_work

Because wake_work and sdio->stat_work share the same workqueue mt76->wq,
if sdio->stat_work cannot acquire the mutex lock such as that was possibly
held up by [mt7615, mt7921]_mutex_acquire. Additionally, if
[mt7615, mt7921]_mutex_acquire was called by sdio->stat_work self, the wake
would be blocked by itself. Thus, we move the stat_work into
ieee80211_workqueue instead to break the deadlock.

Fixes: d39b52e3

 ("mt76: introduce mt76_sdio module")
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>