scsi: target: fix unsigned comparision with less than zero

Currently an error return is being assigned to an unsigned size_t varianle
and then checked if the result is less than zero which will always be
false.  Fix this by making ret ssize_t rather than a size_t.

Fixes: 0322913c ("scsi: target: Add device product id and revision configfs attributes")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>