brcmfmac: sdio unload fix.

on sdio remove the bus_if should be configured for close, so
new data from higher layers will be blocked. Also the access
to bus_if in the watchdog should be checked for null pointer
access on sdio remove.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>