powerpc: wii_defconfig: Disable Ethernet driver support code

The Wii doesn't have built-in Ethernet and USB Ethernet adapters are in
a different menu. Disable CONFIG_ETHERNET to save some space in support
code for Ethernet drivers.

Note that this patch doesn't disable any Ethernet drivers, because they
are not enabled by default.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>