mt76: mt7921: add back connection monitor support

Hw beacon cmd to the mt7921 firmware doesn't only filter out the beacon,
but also performs its own connection monitoring, including periodic
keep-alives to the AP and probing the AP on beacon loss. Will indicate
the host with the event when the firmware detects the connection is lost.

Fixes: 1d8efc74 ("mt76: mt7921: introduce Runtime PM support")
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: YN Chen <yn.chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>