net: dsa: mv88e6060: use the correct MaxFrameSize bit

According to the mv88e6060 datasheet, the MaxFrameSize bit position
is 10 instead of 11 which is reserved.
Use the bit correctly to setup max frame size to 1536.

Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>