brcm80211: fmac: save bus interface structure in function 2 device

bus interface was stored in sdio card device. The device pointer
is used as parameter of interface functions between common layer
and bus layer to make the function declaration generic for different
bus type. But the card device is a parent device layer for SDIO
function devices. It doesn't contain all contexts needed by udev.
This patch moves the shared structure to private driver data pointer
of SDIO function 2 device which is more appopriate for net device
and cfg80211 registration.

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