scsi: ufs: Allow reading descriptor via raw upiu

Allow to read descriptors via raw upiu. This in fact was forbidden just as
a precaution, as ufs-bsg actually enforces which functionality is
supported.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>