greybus: endo: record AP interface id

The AP resides in a particular position on an Endo, which is
identified by an interface ID.  (For now we'll assume the AP uses
just one interface.)  Record the this AP interface ID when creating
an Endo.  Add a sysfs attribute to display it as well.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>