library/GSM_RR_Types.ttcn: fix 'presence' in tr_PacketUlSglAssign

According to 3GPP TS 44.018, section 10.5.2.16 "IA Rest Octets",
the first bit of Packet Uplink Assignment defines whether it is
a dynamic (1) or single block (0) allocation.

Change-Id: If2bee9b1b065fcfedd0e57a6487040cefe2e50c5
1 file changed