brcmfmac: fix invalid ifp lookup in firmware-signalling

The destination entries for firmware-signalled flow control have
the interface id stored. This needs to be translated to bsscfg
index when looking up the ifp object for the interface.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>