brcmfmac: add xtlv support to firmware interface layer

Newer firmware API require commands to use xtlv format. Add support
for that in the firmware interface layer.

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@codeaurora.org>
Link: https://lore.kernel.org/r/1627505434-9544-4-git-send-email-arend.vanspriel@broadcom.com