mt76: mt7921: fix possible resume failure

Fix the possible resume failure due to mt76_connac_mcu_set_hif_suspend
timeout.

That is because clearing the flag pm->suspended too early opened up a race
window, where mt7921_poll_tx/rx scheduled a ps_work to put the device in
doze mode, that is unexpected for the device is being resumed from the
suspend state and would make the remaining MCU comamnds in resume handler
failed to execute.

Fixes: ffa1bf97

 ("mt76: mt7921: introduce PM support")
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>