wcn36xx: Add missing fall through comment in smd.c

This prevents GCC warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>