Refactoring write_packet_ack_nack_desc_egprs to prepare for CRBB support

Change-Id: Ie5c25b6ee30f2f1b613e923c234b03a6ffe12ae2
diff --git a/tests/tbf/TbfTest.err b/tests/tbf/TbfTest.err
index c6bc1aa..38d85d9 100644
--- a/tests/tbf/TbfTest.err
+++ b/tests/tbf/TbfTest.err
@@ -6290,7 +6290,7 @@
 -- Frame 1 starts at offset 0, length=37, is_complete=0
 - No gaps in received block, last block: BSN=1 CV=7
 Encoding Ack/Nack for TBF(TFI=0 TLLI=0xf1223344 DIR=UL STATE=FLOW EGPRS) (final=0)
- - EGPRS URBB, len = 0, SSN = 3, ESN_CRBB = 2, SNS = 2048, WS = 64, V(Q) = 2, V(R) = 2, BOW, EOW
+EGPRS URBB, urbb len = 0, SSN = 3, ESN_CRBB = 2, len present = yes,desc len = 15, SNS = 2048, WS = 64, V(Q) = 2, V(R) = 2, BOW, EOW
 Uplink Ack/Nack bit count 98, max 184, message = 40 24 01 3f 3e 24 46 68 90 87 b0 06 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
 Got RLC block, coding scheme: MCS-3, length: 42 (42))
   UL data: 1d 20 40 c1 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
@@ -6303,7 +6303,7 @@
 TBF(TFI=0 TLLI=0xf1223344 DIR=UL STATE=FLOW EGPRS): data_length=37, data=00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
 - Raising V(R) to 5
 Encoding Ack/Nack for TBF(TFI=0 TLLI=0xf1223344 DIR=UL STATE=FLOW EGPRS) (final=0)
- - EGPRS URBB, len = 2, SSN = 3, ESN_CRBB = 2, SNS = 2048, WS = 64, V(Q) = 2, V(R) = 5, BOW, EOW
+EGPRS URBB, urbb len = 2, SSN = 3, ESN_CRBB = 2, len present = yes,desc len = 17, SNS = 2048, WS = 64, V(Q) = 2, V(R) = 5, BOW, EOW
 Uplink Ack/Nack bit count 100, max 184, message = 40 24 01 3f 3e 24 46 68 90 88 b0 06 8b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
 Got 'TBF(TFI=0 TLLI=0xf1223344 DIR=UL STATE=FLOW EGPRS)', TA=7
 Got MS: TLLI = 0xf1223344, TA = 7