greybus: interface: do not read DME during reactivation

It will soon be possible to reactivate an interface that has been
powered down after already having been activated. In that case there's
no need to re-read the DME attributes as part of activation as the
values are already cached.

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>