Create field filters for gsm_rr_csn_flag() usages.

Change-Id: I77b0c33c054397666f978a9ae175fa386cb87b59
Reviewed-on: https://code.wireshark.org/review/4419
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>