previously we rely on the failsafe setup in preinit scripts to disable
built-in switch implicitly for single-port devices. This doesn't work
anymore due to preinit script removal.
this patch explicitly disable built-in switch for needed devices.
Fixes: a8d62a4eb1 ("ramips: remove set_preinit_iface script")
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>