wl1271: remove sdio ARM dependency

Make it possible to use wl1271's SDIO interface on non-ARM platforms.

Fully tested on a x86 platform, compile-tested on ARM.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>