selftests: mlxsw: qos_pfc: Convert to iproute2 dcb

[ Upstream commit b0bab229 ]

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>
Stable-dep-of: b34f4de6 ("selftests: mlxsw: qos_pfc: Adjust the test to support 8 lanes")
Signed-off-by: Sasha Levin <sashal@kernel.org>