mt76: mt7915: introduce mt7915_mac_severe_check()

In rare cases, TRB pointers might be out of sync leads to RMAC stopping
Rx that requires minimal recovery, so add this helper to periodically
check TRB status.

Tested-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>