brcmfmac: proto: add callback for queuing TX data

So far our core code was calling brcmf_fws_process_skb which wasn't
a proper thing to do. If case of devices using msgbuf protocol fwsignal
shouldn't be used. It was an unnecessary extra layer simply calling
a protocol specifix txdata function.

Please note we already have txdata callback, but it's used for calls
between bcdc and fwsignal so it couldn't be simply used there.

This makes core code more generic (instead of bcdc/fwsignal specific).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>