net: dsa: vsc73xx: make the MTU configurable

Instead of hardcoding the MTU to the maximum value allowed by the
hardware, obey the value known by the operating system.

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>