brcmfmac: add parameter to brcmf_proto_hdrpush() for data offset

The function brcmf_proto_hdrpush() increases the header space and
fills in the protocol header fields. One field is the data offset
which is currently fixed to zero meaning the data follows right
after the header. The parameter is added to determine the actual
start of data. This will be used for firmware signalling.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@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>