blk-mq: fix msec comment from micro to milli seconds

Delay to wait for queue running is milli second unit which is passed to
delayed work via msecs_to_jiffies() which is to convert milliseconds to
jiffies.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>