scsi: ufs-mediatek: Customize WriteBooster flush policy

Change the WriteBooster policy to keep VCC on during runtime suspend if
available WriteBooster buffer is less than 80%.

Link: https://lore.kernel.org/r/20200509093716.21010-5-stanley.chu@mediatek.com


Reviewed-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>