Fix qlogicpti DMA unmapping

[SCSI] QLOGICPTI: Do not unmap DMA unless we actually mapped something.

We only map DMA when cmd->request_bufflen is non-zero for non-sg
buffers, we thus should make the same check when unmapping.

Based upon a report from Pasi Pirhonen.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>