brcm80211: fmac: introduce common bus interface struct brcmf_bus

struct brcmf_bus will contain function porinter, bus specific
private structure pointer and interface context of generic layer
and bus layer. It will be the only shared structure between generic
and bus layer. This patch is part of fullmac bus interface
refactoring.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Alwin Beukers <alwin@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>