wifi: mt76: mt7921: fix channel switch fail in monitor mode

When the chanctx enabled, IEEE80211_CONF_CHANGE_CHANNEL in .config()
would not be called anymore. mt76 cannot control RF channel for monitor.
Add monitor type set-channel flow in .change_chanctx().

Fixes: 41ac53c8 ("wifi: mt76: mt7921: introduce chanctx support")
Reviewed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>