autotest: WiFi: Fix negative config time

Make a parse failure of the returned string from site_wlan_connect
more transparent.  Fix up negative config times and do not emit
them.

BUG=chromium-os:37303,chromium-os:36787
TEST=Run autotest

Change-Id: I8186040297cedeec782c96f589870dd01ad65dac
Reviewed-on: https://gerrit.chromium.org/gerrit/39778
Commit-Ready: Paul Stewart <pstew@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
2 files changed