mt76: mt7921s: fix the device cannot sleep deeply in suspend

According to the MT7921S firmware, the cmd MCU_UNI_CMD_HIF_CTRL have to
be last MCU command to execute in suspend handler and all data traffic
have to be stopped before the cmd MCU_UNI_CMD_HIF_CTRL starts as well
in order that mt7921 can successfully fall into the deep sleep mode.

Where we reuse the flag MT76_STATE_SUSPEND and avoid creating
another global flag to stop all of the traffic onto the SDIO bus.

Fixes: 48fab5bb ("mt76: mt7921: introduce mt7921s support")
Reported-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>