brcmfmac: avoid firmware commands when bus is down

Upon rmmod a few attempts are made to inform firmware, but there is
no point as the bus is down and these will fail. Avoid them to keep
the logs clean.

Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>