mt76: mt7915: enable HE UL MU-MIMO

Enable HE UL MU-MIMO in sta_rec_muru, which works on both ap and station
mode.

For sending trigger frames, one of the conditions fw uses is to check if
mib rx airtime meets the threshold.
There's a main control of mib rx airtime report register in fw, so we
need to enable the register by mcu cmd instead of directly writing it,
otherwise it will still be disabled by fw.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>