brcmfmac: Add definition of new protocol layer msgbuf.

A new protocol layer msgbuf will be added in the future. This
change makes it possible to select the desired layer by the
bus driver. USB and SDIO will select BCDC. At the moment
nothing is being done with this information.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@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>