net: mscc: ocelot: create a helper for changing the port MTU

Since in an NPI/DSA setup, not all ports will have the same MTU, we need
to make sure the watermarks for pause frames and/or tail dropping logic
that existed in the driver is still coherent for the new MTU values.

We need to do this because the NPI (aka external CPU) port needs an
increased MTU for the DSA tag. This will be done in a future patch.

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>