brcmfmac: add definitions for handling sk_buff control buffer data

brcmfmac: add definitions for handling sk_buff control buffer data

The sk_buff structure contains a control buffer that can be used
by different layers in the networking stack for holding packet
associated information. In brcmfmac it is used to hold firmware
signalling related information.

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>