packet-mausb.c: proto-reg-handoff() fixes

- proto_reg_handoff...():
    Fix handling of tcp port pref change;
    Rename 'gPORT_PREF' to 'mausb_tcp_port_pref';
    Do 'dissector_add_uint("llc.mausb_pid",..)' only once;

Change-Id: Ic4498560f61d0ce95f8109c3a1bb3c407221269b
Reviewed-on: https://code.wireshark.org/review/3854
Reviewed-by: Bill Meier <wmeier@newsguy.com>