wifi: brcmfmac: introduce BRCMFMAC exported symbols namespace

Using a namespace variant to make clear it is only intended to be used
by the vendor-specific modules. The symbol will only truly export the
symbols when the driver and consequently the vendor-specific part are
built as kernel modules.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221129135446.151065-8-arend.vanspriel@broadcom.com