brcmfmac: obtain iftype for firmware-signal descriptor lookup

The function brcmf_fws_find_mac_desc() determines the descriptor
associated with a sk_buff for firmware-signalling. It needs the
interface type to do that. For this a helper function is added in
wl_cfg80211.c.

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