selftests: mlxsw: ethtool_lanes: Wait for lanes parameter dump explicitly

The ethtool dump includes the lanes parameter only when the port is up.
Therefore, the ethtool_lanes.sh test waits for ports to come before testing
the lanes parameter.

In some cases, the test considers the port as up, but the lanes parameter
is not yet dumped although assumed to be, resulting in ethtool_lanes.sh
test failure.

To avoid that, ensure that the lanes parameter is indeed dumped by waiting
for it explicitly, before preforming the test cases.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>