[SCSI] bnx2fc: scsi_dma_unmap() not invoked on IO completions

Do not set io_req->sc_cmd to NULL until bnx2fc_unmap_sg_list() is called to
enable it to unmap the DMA mappings.

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