usb: ci_udc: lift ilist size calculations to global scope

This will allow functions other than ci_udc_probe() to make use of the
constants in a future change.

This in turn requires converting the const int variables to #defines,
since the initialization of one global const int can't depend on the
value of another const int; the compiler thinks it's non-constant if
that dependency exists.

Signed-off-by: Stephen Warren <swarren@nvidia.com>