lr-wpan: visibility of constants to upper layers.

In a recent MR !1227 (merged), MAC constants were moved from the .h to the .cc file.

Prior to the change, constants were accessible to upper layers simply by using LrWpanMac::<constant_name>. However, after the change constants are no longer accessible to the upper layers using the mentioned method.

Is also worth mentioning that constants in lower layers are often used to initialize other contantants in higher layers. So whatever change is done, also needs to consider this as well.

Furthermore, !1227 (merged) did not change constants in the PHY and the CSMA/CA which if required, also needs to be moved to keep consistency with the MAC constants changing place.