mtip32xx: convert internal command issue to block IO path

The driver special cases certain things for command issue, depending
on whether it's an internal command or not. Make the internal commands
use the regular infrastructure for issuing IO.

Since this is an 8-group souped up AHCI variant, we have to deal
with NCQ vs non-queueable commands. Do this from the queue_rq
handler, by backing off unless the drive is idle.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>