greybus: audio: return success for stop trigger if device removed

In case GB codec module is already removed, no action is required
at the HW level. Thus, report SUCCESS to above layer.

Reporting error to above layer will cause repeated trials and won't
allow to update DPCM connections.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>