net: mscc: ocelot: split assignment of the cpu port into a separate function

Now that the places that configure routing destinations for the CPU port
have been marked as such, allow callers to specify their own CPU port
that is different than ocelot->num_phys_ports. A user will be the Felix
DSA driver, where the CPU port is one of the physical ports (NPI mode).

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