btmtksdio have to rely on MMC_PM_KEEP_POWER in pm_flags to avoid that
SDIO power is being shut off during the device is in suspend. That fixes
the SDIO command fails to access the bus after the device is resumed.

Fixes: 7f3c563c ("Bluetooth: btmtksdio: Add runtime PM support to SDIO based Bluetooth")
Co-developed-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>