net: mscc: ocelot: don't use NPI tag prefix for the CPU port module

Context: Ocelot switches put the injection/extraction frame header in
front of the Ethernet header. When used in NPI mode, a DSA master would
see junk instead of the destination MAC address, and it would most
likely drop the packets. So the Ocelot frame header can have an optional
prefix, which is just "ff:ff:ff:ff:ff:fe > ff:ff:ff:ff:ff:ff" padding
put before the actual tag (still before the real Ethernet header) such
that the DSA master thinks it's looking at a broadcast frame with a
strange EtherType.

Unfortunately, a lesson learned in commit 69df578c

 ("net: mscc:
ocelot: eliminate confusion between CPU and NPI port") seems to have
been forgotten in the meanwhile.

The CPU port module and the NPI port have independent settings for the
length of the tag prefix. However, the driver is using the same variable
to program both of them.

There is no reason really to use any tag prefix with the CPU port
module, since that is not connected to any Ethernet port. So this patch
makes the inj_prefix and xtr_prefix variables apply only to the NPI
port (which the switchdev ocelot_vsc7514 driver does not use).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>