Add support for communicating with a Sonics Silicon Backplane through a
SDIO interface, as found in the Nintendo Wii WLAN daughter card.

The Nintendo Wii WLAN card includes a custom Broadcom 4318 chip with
a SDIO host interface.
Signed-off-by: NAlbert Herranz <albert_herranz@yahoo.es>
Signed-off-by: NMichael Buesch <mb@bu3sch.de>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>