tbf/bts, encoding: Keep track of WAIT_RELEASE state for DL assignment

The current code does not properly distinguish between DL assignments to
reuse a tbf (after it was put in state WAIT_RELEASE) and DL assignments
for an active tbf to change the allocation of the PDCH timeslots.

This patch introduces a new variable was_releasing which remembers if
trigger_dl_ass() was called with a tbf in state WAIT_RELEASE. In that
case we have to set the CONTROL_ACK field in the download assignment.

This should allow us to send DL assignments to change PDCH TS allocation
of a tbf before we enter FLOW state.
diff --git a/src/encoding.cpp b/src/encoding.cpp
index 51b0fbe..13848aa 100644
--- a/src/encoding.cpp
+++ b/src/encoding.cpp
@@ -267,7 +267,7 @@
 
 	block->u.Packet_Downlink_Assignment.MAC_MODE            = 0x0;          // Dynamic Allocation
 	block->u.Packet_Downlink_Assignment.RLC_MODE            = 0x0;          // RLC acknowledged mode
-	block->u.Packet_Downlink_Assignment.CONTROL_ACK         = old_downlink; // NW establishes no new DL TBF for the MS with running timer T3192
+	block->u.Packet_Downlink_Assignment.CONTROL_ACK         = tbf->was_releasing; // NW establishes no new DL TBF for the MS with running timer T3192
 	block->u.Packet_Downlink_Assignment.TIMESLOT_ALLOCATION = 0;   // timeslot(s)
 	for (tn = 0; tn < 8; tn++) {
 		if (tbf->pdch[tn])