block: use right format specifier for op

In function __blk_mq_debugfs_rq_show variable op has unsigned int type.
Since op can never be negative use %u format specifier to match the
variable type.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>