mt76: connac: introduce mt76_connac_mcu_set_deep_sleep utility

Introduce mt76_connac_mcu_set_deep_sleep to enable deep sleep mode
and will be activated immediately when the host returns the ownership
to the device.

Co-developed-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Co-developed-by: YN Chen <YN.Chen@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>