net: mscc: ocelot: export a constant for the tag length in bytes

This constant will be used in a future patch to increase the MTU on NPI
ports, and will also be used in the tagger driver for Felix.

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