/bin/wifi: call ifup/ifdown.

In addition to setting the link state, ifdown also kills dhclient.

Change-Id: I889f2d252f9fb2bcb8b9141c5156ca0c506257aa
diff --git a/wifi/quantenna.py b/wifi/quantenna.py
index 7aad0d0..f3f96ef 100755
--- a/wifi/quantenna.py
+++ b/wifi/quantenna.py
@@ -54,7 +54,7 @@
 
 
 def _set_link_state(hif, state):
-  subprocess.check_output(['ip', 'link', 'set', 'dev', hif, state])
+  subprocess.check_output(['if' + state, hif])
 
 
 def _ifplugd_action(hif, state):