scsi: ufs: don't check unsigned type for a negative value

Fix compilation warning:

drivers/scsi/ufs/ufshcd.c:7645:13: warning: comparison of unsigned
expression < 0 is always false [-Wtype-limits]
if ((value < UFS_PM_LVL_0) || (value >= UFS_PM_LVL_MAX))

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>