scsi: scsi_dh_alua: Fix signedness bug in alua_rtpg()

The "retval" variable needs to be signed for the error handling to work.

Link: https://lore.kernel.org/r/YLjMEAFNxOas1mIp@mwanda
Fixes: 7e26e3ea

 ("scsi: scsi_dh_alua: Check for negative result value")
Reviewed-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>