[SCSI] bnx2fc: Handle bnx2fc_map_sg failure

Gracefully handle bnx2fc_map_sg failure, so that queuecommand returns host busy
and SCSI-ml can retry the IO.

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>