lpfc: Fix scsi prep dma buf error.

lpfc: Fix scsi prep dma buf error.

Didn't check for less-than-or-equal zero. Means we may later call
scsi_dma_unmap() even though we don't have valid mappings.

Signed-off-by: Dick Kennedy <dick.kennedy@avagotech.com>
Signed-off-by: James Smart <james.smart@avagotech.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>
(cherry picked from commit 5116fbf136ea21b8678a85eee5c03508736ada9f)

Orabug: 22029622
From dick.kennedy@avagotech.com lpfc-10.5.0.1-11.0.0.3-1.tar.gz
Acked-by: Chuck Anderson <chuck.anderson@oracle.com>