Eliminate some proto_tree_add_text in packet-bgp.c

Change-Id: I05fabe9786125e2167fe4010733f61000d0e7914
Reviewed-on: https://code.wireshark.org/review/5670
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Matthieu Texier <matthieu@texier.tv>
Reviewed-by: Anders Broman <a.broman58@gmail.com>