mt76: mt7615: set mcu country code in mt7615_mcu_set_channel_domain()

Update mcu country code running mt7615_mcu_set_channel_domain routine in
mt7615_regd_notifier().
Filter out disabled channels in mt7615_mcu_set_channel_domain().

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>