greybus: svc: refactor interface-route destruction

Add interface-route-destroy helper to tear down the route and release
the interface device id.

Note that we currently need to grab a reference to the interface to
prevent it from being deallocated before tearing down the route.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>