wifi: mt76: mt7915: accumulate mu-mimo ofdma muru stats

The stats are clear-on-read, which makes it very difficult for tools
to adequately deal with wrapped stats and with keeping good totals.

So, accumulate these values when they are read from the firmware/radio
and present totals to user-space.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>