brcmfmac: On bus flow control use fw signalling or netif.

Currently on a bus flow control both fws is informed and netif queue
gets closed. In case of fw signalling enabled, let the flow control
be handled by fw signalling only.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>