brcmfmac: add flow-control mode to firmware signalling

Upcoming patches will add firmware signalled flow control. Prepare
by adding the mode, which defaults to disable it. The mode can be
queried by brcmf_fws_fc_active() and set by a module parameter.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>