realtek rtl931x: mark subtarget as source-only
There are no supported devices on this sub-target. It can be
considered that it is still under development. Therefore,
there is no need to make the buildbot build it every day.
Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18757
Signed-off-by: Robert Marko <[email protected]>
Shiji Yang