Use proto_tree_add_bitmask for ExpandedNodeId EncodingMask

Instead of adding the items using proto_tree_add_item, use proto_tree_add_bitmask in parseExpandedNodeId(). Also, the redundant 'NodeId ' text is removed from hf_opcua_nodeid_... items for better readability.

Change-Id: Ie68f1d280dd733fd2eede0b2b73ad8d7f28396b3
Reviewed-on: https://code.wireshark.org/review/4730
Reviewed-by: Michael Mann <mmann78@netscape.net>