scsi: ufs: Fix NULL pointer dereference in ufshcd_config_vreg_hpm()

Fix the following BUG:

  [ 187.065689] BUG: kernel NULL pointer dereference, address: 000000000000001c
  [ 187.065790] RIP: 0010:ufshcd_vreg_set_hpm+0x3c/0x110 [ufshcd_core]
  [ 187.065938] Call Trace:
  [ 187.065959] ufshcd_resume+0x72/0x290 [ufshcd_core]
  [ 187.065980] ufshcd_system_resume+0x54/0x140 [ufshcd_core]
  [ 187.065993] ? pci_pm_restore+0xb0/0xb0
  [ 187.066005] ufshcd_pci_resume+0x15/0x20 [ufshcd_pci]
  [ 187.066017] pci_pm_thaw+0x4c/0x90
  [ 187.066030] dpm_run_callback+0x5b/0x150
  [ 187.066043] device_resume+0x11b/0x220

Voltage regulators are optional, so functions must check they exist
before dereferencing.

Note this issue is hidden if CONFIG_REGULATORS is not set, because the
offending code is optimised away.

Notes for stable:

The issue first appears in commit 57d104c1 ("ufs: add UFS power
management support") but is inadvertently fixed in commit 60f01870
("scsi: ufs: disable vccq if it's not needed by UFS device") which in
turn was reverted by commit 73067981 ("Revert "scsi: ufs: disable vccq
if it's not needed by UFS device""). So fix applies v3.18 to v4.5 and
v5.1+

Fixes: 57d104c1 ("ufs: add UFS power management support")
Fixes: 73067981

 ("Revert "scsi: ufs: disable vccq if it's not needed by UFS device"")
Cc: stable@vger.kernel.org
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>