net: mscc: ocelot: make the NPI port a proper target for FDB and MDB

When used in DSA mode (as seen in Felix), the DEST_IDX in the MAC table
should point to the PGID for the CPU port (PGID_CPU) and not for the
Ethernet port where the CPU queues are redirected to (also known as Node
Processor Interface - NPI).

Because for Felix this distinction shouldn't really matter (from DSA
perspective, the NPI port _is_ the CPU port), make the ocelot library
act upon the CPU port when NPI mode is enabled. This has no effect for
the mscc_ocelot driver for VSC7514, because that does not use NPI (and
ocelot->npi is -1).

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