scsi: mvumi: remove code handling zero scsi_sg_count(scmd) case

As Christoph Hellwig noted, SCSI commands that transfer data always have
a SG entry. The patch removes dead code in mvumi_make_sgl(),
mvumi_complete_cmd() and mvumi_timed_out() that handle zero
scsi_sg_count(scmd) case.

Also the patch adds pci_unmap_sg() on failure path in mvumi_make_sgl().

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>