net/sched: taprio: don't overwrite "sch" variable in taprio_dump_class_stats()

In taprio_dump_class_stats() we don't need a reference to the root Qdisc
once we get the reference to the child corresponding to this traffic
class, so it's okay to overwrite "sch". But in a future patch we will
need the root Qdisc too, so create a dedicated "child" pointer variable
to hold the child reference. This also makes the code adhere to a more
conventional coding style.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>