greybus: interface: prevent reactivation during removal

Make sure to prevent an interface that is going away from being
reactivated.

This is needed to preemptively close a race between the upcoming feature
to reactivate a powered-down interface and physical removal (i.e.
module_removed event processing) as well as logical removal (e.g. the
current system-suspend hack).

Reviewed-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>