selftests: mlxsw: qos_headroom: Convert to iproute2 dcb

There is a dedicated tool for configuration of DCB in iproute2 now. Use it
in the selftest instead of mlnx_qos.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>