mt76: mt7921: enable deep sleep at runtime

Enable the deep sleep mode with that firmware is able to trap into
the doze state at runtime to reduce the power consumption further.

The deep sleep mode is not allowed in the STA state transition with
the firmware to have the fast connection experience as we've done in
the full power mode

Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>