tbf: Store the timing advance (TA) value in the GprsMs object

The TA value rather relates to an MS and not to a single TBF. So all
TBFs share the same TA value. Currently the TA value is stored per
TBF and eventually copied from an old TBF to a new one. It is in
general only passed with an RACH request when the TLLI and thus the
MS is not yet known.

This commit adds a TA member to the GprsMs class and uses that one
when the TBF is associated to an MS object. Since the TBF is not
always associated with an MS object (after RACH or when it has been
replaced by another TBF), the TA value is still stored in each TBF
and that value is used as long as no MS object is being associated.

Sponsored-by: On-Waves ehf
diff --git a/src/encoding.cpp b/src/encoding.cpp
index ffd6108..167bfd4 100644
--- a/src/encoding.cpp
+++ b/src/encoding.cpp
@@ -195,7 +195,7 @@
 	bitvec_write_field(dest, wp,bts->initial_cs_ul-1, 2); // CHANNEL_CODING_COMMAND 
 	bitvec_write_field(dest, wp,0x1,1); // TLLI_BLOCK_CHANNEL_CODING 
 	bitvec_write_field(dest, wp,0x1,1); // switch TIMING_ADVANCE_VALUE = on
-	bitvec_write_field(dest, wp,tbf->ta,6); // TIMING_ADVANCE_VALUE
+	bitvec_write_field(dest, wp,tbf->ta(),6); // TIMING_ADVANCE_VALUE
 	if (ta_idx < 0) {
 		bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
 	} else {
@@ -275,7 +275,7 @@
 	}
 
 	block->u.Packet_Downlink_Assignment.Packet_Timing_Advance.Exist_TIMING_ADVANCE_VALUE = 0x1; // TIMING_ADVANCE_VALUE = on
-	block->u.Packet_Downlink_Assignment.Packet_Timing_Advance.TIMING_ADVANCE_VALUE       = tbf->ta;  // TIMING_ADVANCE_VALUE
+	block->u.Packet_Downlink_Assignment.Packet_Timing_Advance.TIMING_ADVANCE_VALUE       = tbf->ta();  // TIMING_ADVANCE_VALUE
 	if (ta_idx < 0) {
 		block->u.Packet_Downlink_Assignment.Packet_Timing_Advance.Exist_IndexAndtimeSlot     = 0x0; // TIMING_ADVANCE_INDEX = off
 	} else {