mt76: mt7921: fix up the monitor mode

Properly set up the monitor mode the firmware can support to fix up
RTS/CTS and beacon frames cannot be captured and forwarded to the host.

Tested-by: Deren Wu <deren.wu@mediatek.com>
Tested-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>