selftests: mlxsw: qos_pfc: Remove wrong description

In the diagram of the topology, $swp3 and $swp4 are described as 1Gbps
ports. This is wrong information, the test does not configure such speed.

Cc: Shuah Khan <shuah@kernel.org>
Fixes: bfa80478 ("selftests: mlxsw: Add a PFC test")
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/0087e2d416aff7e444d15f7c2958fc1d438dc27e.1705502064.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>