At the moment, oFono/MM are usually stopped after rmtfs on shutdown.
This does not make sense, since the modem managed by oFono/MM will
disappear once rmtfs is stopped. Add a "before ofono modemmanager"
to make sure that OpenRC will stop oFono/MM before rmtfs on shutdown.

This may or may not fix some occasional hangs during shutdown,
but it definitely makes it possible for oFono/MM to shutdown cleanly.