brcm80211: fmac: abstract bus_stop interface function pointer

Common layer should use interface function pointer stored in
brcmf_bus to invoke corresponding interface function in bus layer.
This patch 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>