wifi: mt76: mt7921: fix deadlock in mt7921_abort_roc

When mt7921_abort_roc is called with dev->mutex held, it can deadlock while
calling cancel_work_sync(&phy->roc_work), because the work function could
be waiting to acquire the mutex.

Fix this by flushing the work before taking the mutex

Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Fixes: 034ae28b ("wifi: mt76: mt7921: introduce remain_on_channel support")
Fixes: 41ac53c8 ("wifi: mt76: mt7921: introduce chanctx support")
Signed-off-by: Felix Fietkau <nbd@nbd.name>