wcn36xx: Fix up wcn36xx_smd_update_scan_params()

Fix up the wcn36xx_smd_update_scan_params() to work with non-ancient
versions of the firmware and support actually specifying the list of
channels.

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