net: mscc: ocelot: use a switch-case statement in ocelot_netdevice_event

Make ocelot's net device event handler more streamlined by structuring
it in a similar way with others. The inspiration here was
dsa_slave_netdevice_event.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>