3w-xxxx: fix command completion race

The 3w-xxxx driver needs to tear down the dma mappings before returning
the command to the midlayer, as there is no guarantee the sglist and
count are valid after that point.  Also remove the dma mapping helpers
which have another inherent race due to the request_id index.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>