scsi: pmcraid: remove redundant check to see if request_size is less than zero

The 2nd check to see if request_size is less than zero is redundant
because the first check takes error exit path on this condition. So,
since it is redundant, remove it.

Detected by CoverityScan, CID#146149 ("Logically Dead Code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>