Be lenient if deconfig fails to find a bridge

If we are in the deconfig step, it is acceptable if brctl
cannot find the bridge it was trying to delete.

BUG=none
TEST=run_wifi_tests on SecMat

Review URL: http://codereview.chromium.org/3387009
diff --git a/server/site_linux_router.py b/server/site_linux_router.py
index 39c39a1..2405fe5 100644
--- a/server/site_linux_router.py
+++ b/server/site_linux_router.py
@@ -319,7 +319,7 @@
             result = self.router.run("%s delbr %s" %
                                      (self.cmd_brctl, self.bridgeif),
                                      ignore_status=True)
-            if not result.stderr:
+            if not result.stderr or 'No such device' in result.stderr:
                 break
             time.sleep(1)
         else: