wcn36xx: Implement multicast filtering

Pass the multicast list to FW.

This patch also adds a way to build the smd command in place. This is
needed because the MC list command is too big for the stack.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
[bjorn: dropped FIF_PROMISC_IN_BSS usage]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>