mt76: mt7663s: fix resume failure

MT7663s have to rely on MMC_PM_KEEP_POWER in pm_flags for to avoid SDIO
power is being shut off.

To fix sdio access failure like "mt7663s mmc1:0001:1: sdio write failed:
-22" for the first sdio command to access the bus in the resume handler.

Fixes: a66cbdd6 ("mt76: mt7615: introduce mt7663s 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>