[SCSI] aacraid: SCSI dma mapping failure case handling

This patch handles SCSI dma mapping failure case. Reporting error code to the
upper layer instead of BUG_ON().

Signed-off-by: Mahesh Rajashekhara <Mahesh_Rajashekhara@pmc-sierra.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>