[SCSI] mpt2sas: fix the incorrect scsi_dma_map error checking

scsi_dma_map() returns -1 if an error occurred (zero means that the
command has no data). So the following current code can't catch an
error:

sges_left = scsi_dma_map(scmd);
if (!sges_left) {
	sdev_printk(KERN_ERR, scmd->device, "pci_map_sg"
	" failed: request for %d bytes!\n", scsi_bufflen(scmd));
	return -ENOMEM;
}

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: "Desai, Kashyap" <Kashyap.Desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>