mtip32xx: move the blk_rq_map_sg call to mtip_hw_submit_io

We have all arguments at hand in mtip_hw_submit_io, so keep the
rq to sg mapping close to the dma_map_sg call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>