lr-wpan: Change CapabilityField to standard bitmap

This MR is sort of a follow-up to !1604 (merged).

It changes the CapabilityField parameter of some primitives to the standard bitmap.

Internally, these classes will still use the CapabilityField for easy handling. However, the change to the parameter is necessary to keep the parameters compliant with what the standard describes and what higher layers expect.

The change is on primitives LrWpanMac::MlmeAssociateRequest and LrWpanMac::MlmeAssociateIndication other changes are a consequence of these changes.

Release notes/changes will be added before the final merge.