brcm80211: fmac: move busstate to struct brcmf_bus

busstate keeps track of the bus (USB/SDIO) status and is used by
both generic layer and bus layer. Move it to brcmf_bus helps to clean
up the interface. 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>