greybus: audio: Report warning in case module is already removed

Added warning message in find_gb_module(). This will help to identify
invalid mixer control/widget modification triggered from above layer.

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>