Cleanup packet-sccp.c in preparation of trying to remove sccp_info member of packet_info.

Convert a handful of global variables into function parameters that get passed through the dissector, so we shouldn't be dependent on pinfo->sccp_info anymore. Removal of pinfo->sccp_info will be done in a separate patch (when the dissectors that use it can be updated).

packet-sua.c may need similar treatment.

Change-Id: If0001638d666afc07e04b02aa32ef31d6223a5de
Reviewed-on: https://code.wireshark.org/review/5343
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>