scsi: ufs: fix crash upon reading host_regs debugfs entry

While reading host_regs debugfs entry, UFS controller
might be in low power mode. While in UFS runtime suspend mode the host
controller is being unclocked.
Reading the unclocked host controller registers, may cause a system crash.
In order to avoid that, UFS runtime suspend should be
held and released when done.

CRs-fixed: 594618
Change-Id: I4b850806352b59192340bc5b0bb480b4dd6553f1
Signed-off-by: Raviv Shvili <rshvili@codeaurora.org>