greybus: endo: fix device-id allocation

During endo registration, a unique device id was allocated but then
never used. Instead the default dev_id 0 (due to kzalloc) was used for
device-generation and later for id-deallocation.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>