greybus: remove serial number descriptor to match spec

Greybus spec was updated to remove the serial number descriptor and
move the serial number field to the, now mandatory, module descriptor.
Change everything accordingly.

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