selftests: mlxsw: qos_defprio: Use until_counter_is

Instead of hand-coding the busywait() predicate, use the until_counter_is()
introduced recently.

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