mt76: mt7915: add mu-mimo and ofdma debugfs knobs

Add mu-mimo and ofdma packet counters statistics.
The statistics are clear on read.

Reviewed-by: Money Wang <Money.Wang@mediatek.com>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>