brcmfmac: increase required skbuff headroom for firmware signalling

In preparation of firmware signalling feature additional headroom is
needed to accommodate signalling protocol data between host and
firmware.

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