lpfc: Fix scsi prep dma buf error.

[ Upstream commit 5116fbf136ea21b8678a85eee5c03508736ada9f ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>