selftests: forwarding: tc_common: Convert to use busywait

A function busywait() was recently added based on the logic in
__tc_check_packets(). Convert the code in tc_common to use the new
function.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>