greybus: fix gb_add_module() by enabling the device_add()

Without the gb_module device being added, we have no parent
device for any of the greybus subdevs to be added. Do the
device_add() before creating subdevs as we need it then
to register any children in the various greybus protocol
drivers.

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