UPSTREAM: 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>

Bug: 151050916
(cherry picked from commit f48b285ae658b50f4ba2d3a267522f572720de6d)
Change-Id: I552f19b2806b5819a49ffff02768c2a5afa4cec7
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>