block: code cleanup queue_poll_stat_show()

This is a pure code cleanup patch and doesn't change any functionality.
Having multiple coding styles in the code creates confusion when
someone tries to add a new code.

Make queue_poll_stat_show() consistent by adding spaces around binary
operators with the rest of the code.

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