UPSTREAM: scsi: ufs-mediatek: Prevent LPM operation on undeclared VCC

In some platforms, VCC regulator may not be declared in device tree to keep
itself "always-on". In this case, hba->vreg_info.vcc is NULL and shall not
be operated during any flow.

Prevent possible NULL hba->vreg_info.vcc access in LPM mode by checking
if it is valid first.

Link: https://lore.kernel.org/r/20200724141627.20094-1-stanley.chu@mediatek.com


Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Bug: 166747994
(cherry picked from commit 0255b1e3d84962f8f8d19f9e2768a8a4f0de7487)
Change-Id: I6351c0bb93ba78a66e286e6aaf8a0c8956cab895
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>