wcn36xx: Transition driver to SMD client

The wcn36xx wifi driver follows the life cycle of the WLAN_CTRL SMD
channel, as such it should be a SMD client. This patch makes this
transition, now that we have the necessary frameworks available.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>