A helper for checking whether a net_device belongs to mscc_ocelot
already existed and did not need to be rewritten. Use it.

Fixes: 319e4dd1 ("net: mscc: ocelot: introduce conversion helpers between port and netdev")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20201011092041.3535101-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>