[SCSI] uas: disable use of blk-mq I/O path

The uas driver uses the block layer tag for USB3 stream IDs.  With
blk-mq we can get larger tag numbers that the queue depth, which breaks
this assumption.  A fix is under way for 3.18, but sits on top of
large changes so can't easily be backported.   Set the disable_blk_mq
path so that a uas device can't easily crash the system when using
blk-mq for SCSI.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>