greybus: endo: encapsulate computing the max interface id

The maximum interface id on an Endo is the result of a non-trivial
calculation.  It'll be needed for an upcoming patch, so create a
macro to compute it.

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