brcmfmac: wrap brcmf_fws_add_interface into bcdc layer

fwsignal is only used by bcdc. Create a new protocol interface function
brcmf_proto_add_if for core module to notify protocol layer upon a new
interface is created.

Signed-off-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>