mt76: mt7921: fix dma hang in rmmod

mt76: mt7921: fix dma hang in rmmod

[ Upstream commit a23f80aa9c5e6ad4ec8df88037b7ffd4162b1ec4 ]

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: 2b9ea5a8cf1bd ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>