blk-mq: make __blk_mq_stop_hw_queues static

Making __blk_mq_stop_hw_queues static fixes sparse warning:

  block/blk-mq.c:6: warning: symbol '__blk_mq_stop_hw_queues' was not
  declared. Should it be static?

Fixes: 2719aa21

 ("blk-mq: don't use sync workqueue flushing from drivers")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>