scsi: ufs: host: mediatek: Fix device power control

Adjust the timing of device power control to ensure low power mode (LPM)
is entered only after VCC is turned off. Prevent VCCQ/VCCQ2 from
entering LPM prematurely, ensuring proper power management and device
stability.

Signed-off-by: Peter Wang <peter.wang@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>