powerpc: wii_defconfig: Disable BCMA support

The B43 driver only needs CONFIG_SSB to support the WLAN card found in
the Wii. Configure it accordingly, and disable BCMA bus support to save
a bit of space.

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