brcm80211: fmac: move dongle statistics to struct brcmf_bus

Dongle statistics are shared data between common layer and bus
layer. This patch places them in bus interface structure brcmf_bus
as 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>