net: mscc: ocelot: adjust MTU on the CPU port in NPI mode

When using the NPI port, the DSA tag is passed through Ethernet, so the
switch's MAC needs to accept it as it comes from the DSA master. Increase
the MTU on the external CPU port to account for the length of the
injection header.

Without this patch, MTU-sized frames are dropped by the switch's CPU
port on xmit, which is especially obvious in TCP sessions.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>