net: dsa: felix: add functionality when not all ports are supported

When the Felix driver would probe the ports and verify functionality, it
would fail if it hit single port mode that wasn't supported by the driver.

The initial case for the VSC7512 driver will have physical ports that
exist, but aren't supported by the driver implementation. Add the
OCELOT_PORT_MODE_NONE macro to handle this scenario, and allow the Felix
driver to continue with all the ports that are currently functional.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # regression
Signed-off-by: Jakub Kicinski <kuba@kernel.org>