82xx, mgcoge: fix compile error

With actual u-boot compiling the mgcoge port fails, because
since commit ba705b5b it is
necessary to define CONFIG_NET_MULTI.

Seems to me the mgcoge port is the only actual existing 8260
port who uses CONFIG_ETHER_ON_SCC, so no other 8260 port needed
to be fixed.

Signed-off-by: Heiko Schocher <hs@denx.de>