brcmfmac: ignore interfaces when fwsignal is disabled

When brcmf_fws_add_interface() is called the struct brcmf_if::fws_desc
field is initialized regardless the state of the fwsignal functionality,
ie. the fcmode. This is not needed when fcmode is NONE, which is the
default mode.

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>