mt76: mt7921: fix stats register definitions

Fix register definitions for mac80211 stats reporting.
Move mib counter reset to mt7921_get_stats routine.

Fixes: 163f4d22 ("mt76: mt7921: add MAC support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>