mtip32xx: abstract out "are any commands active" helper

This is a prep patch for backoff in ->queue_rq() for non-ncq commands.

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