module-loopback: Never call adjust_rates after teardown

module-loopback: Never call adjust_rates after teardown

Calling adjust_rates after teardown results in segfault, and
judging from the Ubuntu bug report, this can happen.
Actively prevent this by destroying the time event, and by
setting adjust_time to 0, we also prevent this routine being
called on max request update.

BugLink: https://bugs.launchpad.net/bugs/946400
Signed-off-by: David Henningsson <david.henningsson@canonical.com>