autotest: Fix IBSS interface teardown

This fixes network_WiFi_IBSS, which was failing during router teardown.

BUG=chromium:323361
TEST=network_WiFi_IBSS passes.

Change-Id: I70a0b56500d3ac3bcfc4a37929cbc47f518e6fb2
Reviewed-on: https://chromium-review.googlesource.com/178127
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
1 file changed