mt76: mt7921: fix dma hang in rmmod

The dma would be broken after rmmod flow. There are two different
cases causing this issue.
1. dma access without privilege.
2. hw access sequence borken by another context.

This patch handle both cases to avoid hw crash.

Fixes: 2b9ea5a8 ("mt76: mt7921: add mt7921_dma_cleanup in mt7921_unregister_device")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>