Previously switching to non-existing device (interface) could result in
leaving LED on.
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Backport of r47990
git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@47992 3c298f89-4303-0410-b956-a3cf2f4a3e73