brcm80211: fmac: use specific types in struct brcmf_bus

The fields bus_priv and drvr are defined as void pointer. It is
preferred to have specific types for compiler type checking. To
prepare for other bus types the bus_priv field is defined as a
union containing the sdio bus private structure reference.

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