net: dsa: Define .set_max_frame_size() callback for mv88e6250 SoC family

Switches from mv88e6250 family (including mv88e6020 and mv88e6071) need
the possibility to setup the maximal frame size, as they support frames
up to 2048 bytes.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>