[SCSI] u14-34f: fix scsi_dma_map failure case

When unsigned, scsi_dma_map may return -ENOMEM without triggering BUG_ON()

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>