selftests: mlxsw: qos_defprio: Convert from lldptool to dcb

Set up default port priority through the iproute2 dcb tool, which is easier
to understand and manage.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>