brcm80211: fmac: move common layer bus interface context to dhd_bus.h

dhd_bus.h is the header file for bus interface. Move functions
declarations and brcmf_bus structure to there. This is part of
the 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>