net: gemini: Indicate that we can handle jumboframes

The hardware supposedly handles frames up to 10236 bytes and
implements .ndo_change_mtu() so accept 10236 minus the ethernet
header for a VLAN tagged frame on the netdevices. Use
ETH_MIN_MTU as minimum MTU.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>