brcmfmac: only build fwsignal module for CONFIG_BRCMFMAC_PROTO_BCDC

The fwsignal module is only referenced by the bcdc module and part of the
bcdc protocol. So only build it when CONFIG_BRCMFMAC_PROTO_BCDC is selected.

Fixes: acf8ac41 ("brcmfmac: remove reference to fwsignal data from struct brcmf_pub")
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>