autotest: Fix logging in network_WiFi_BgscanBackoff

Now when the test fails, it will display something meaningful rather
than failing because key1 is undefined.

TEST=This test passes, meaningful output is printed.
BUG=None

Change-Id: I0d0a6cac78a4a77761aaaab7682dcbfcfd4d1aef
Reviewed-on: https://gerrit.chromium.org/gerrit/58973
Reviewed-by: Tan Gao <tgao@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
1 file changed