net: mscc: ocelot: make phy_mode a member of the common struct ocelot_port

The Ocelot switchdev driver and the Felix DSA one need it for different
reasons. Felix (or at least the VSC9959 instantiation in NXP LS1028A) is
integrated with the traditional NXP Layerscape PCS design which does not
support runtime configuration of SerDes protocol. So it needs to
pre-validate the phy-mode from the device tree and prevent PHYLINK from
attempting to change it. For this, it needs to cache it in a private
variable.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>