greybus: implement core module removal path

Implement gb_remove_module() by finding the gb_module to
be removed via the supplied module_id. Add support for
removing the actual device into greybus_remove_device()
after all the subdevs are disconnected.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>