selftests: lib: tc_rule_stats_get(): Move default to argument definition

The argument $dir has a fallback value of "ingress". Move the fallback from
the usage site to the argument definition block to make the fact clearer.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>