tbf: Use TLLI as ID if TFI not yet assigned

Currently the old TFI is always used as ID when a
PACKET DOWNLINK ASSIGNMENT is generated. This fails
if the old TBF has not been fully assigned yet. The
MS will then ignore the PDA.

This commit changes write_packet_downlink_assignment to accept
an additional parameter old_tfi_is_valid and uses the new TBF's
TLLI instead of the olf TFI if that parameter is set to false.

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.err b/tests/tbf/TbfTest.err
index 177a5b5..02b6a3d 100644
--- a/tests/tbf/TbfTest.err
+++ b/tests/tbf/TbfTest.err
@@ -58,6 +58,7 @@
 The MS object cannot fully confirm an unexpected TLLI: 0xffeeddcc, partly confirmed
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
+New and old TBF are the same TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)  start Packet Downlink Assignment (PACCH)
 +++++++++++++++++++++++++ TX : Packet Downlink Assignment +++++++++++++++++++++++++
 ------------------------- TX : Packet Downlink Assignment -------------------------
@@ -140,6 +141,7 @@
 The MS object cannot fully confirm an unexpected TLLI: 0xffeeddcc, partly confirmed
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
+New and old TBF are the same TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)  start Packet Downlink Assignment (PACCH)
 +++++++++++++++++++++++++ TX : Packet Downlink Assignment +++++++++++++++++++++++++
 ------------------------- TX : Packet Downlink Assignment -------------------------
@@ -222,6 +224,7 @@
 The MS object cannot fully confirm an unexpected TLLI: 0xffeeddcc, partly confirmed
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW) append
+New and old TBF are the same TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)
 TBF(TFI=0 TLLI=0xffeeddcc DIR=DL STATE=FLOW)  start Packet Downlink Assignment (PACCH)
 +++++++++++++++++++++++++ TX : Packet Downlink Assignment +++++++++++++++++++++++++
 ------------------------- TX : Packet Downlink Assignment -------------------------