mwifiex: ensure "disable auto DS" struct is initialized

The .idle_time field *should* be unused, but technically, we're allowing
unitialized stack garbage to pass all the way through to the firmware
host command. Let's zero it out instead.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>