greybus: interface: fix interface_id attribute group

All interfaces, regardless of type, should have an interface_id
attribute reflecting its position on the frame.

This has been reported to cause an assertion failure in libmoduleutil
for dummy modules.

Testing done: Verified that the attribute is present for registered
dummy interfaces.

Reported-by: Gjorgji Rosikopulos <grosikopulos@mm-sol.com>
Tested-by: Gjorgji Rosikopulos <grosikopulos@mm-sol.com>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>