mtip32xx: merge mtip_submit_request into mtip_queue_rq

mtip32xx: merge mtip_submit_request into mtip_queue_rq

Factor out a new is_stopped helper that matches the existing
is_se_active helper, and merge the trivial amount of remaining code
into the only caller.  This also allows better error handling by
returning a BLK_STS_* directly instead of explicitly calling
blk_mq_end_request, and moving blk_mq_start_request closer to the
actual issue to hardware.

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