brcm80211: fmac: add Kconfig option for SDIO bus support

This patch adds a Kconfig option for SDIO bus support and abstracts
a build subset correspondingly. It's the final patch of fullmac bus
interface refactoring.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>