wifi: mt76: mt7921: introduce remain_on_channel support

Introduce remain_on_channel support. Additionally, we add
mt7921_check_offload_capability to disable .remain_on_channel and
.cancel_remain_on_channel and related configuration because those
operations would rely on the fundamental MCU commands that will be only
supported with newer firmware.

Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>