mt76: mt7915: fix beamforming mib stats

Some of beamforming mib counters are moved to different offsets
or registers in newer chipsets.

Fixes: 99ad32a4

 ("mt76: mt7915: add support for MT7986")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>