bql: Fix inconsistency between file mode and attr method.

There is no store() method for inflight attribute in the
tx-<n>/byte_queue_limits sysfs directory.
So remove S_IWUSR bit.

Signed-off-by: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>