pdch_ulc: Store TBF poll reason

This allows easily checking the initial reason to trigger the poll when
either it is received or times out.

Later on this reason can be transformed into an FSM event and sent to
the related FSM.

Related: OS#5020
Change-Id: Ie8fefd1f47ad674ce597a8065b15284088956bde
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 44baa00..a59b03b 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -975,7 +975,7 @@
 
 		rc = check_polling(fn, ts, &new_poll_fn, &rrbp);
 		if (rc >= 0) {
-			set_polling(new_poll_fn, ts, GPRS_RLCMAC_POLL_DL_ACK);
+			set_polling(new_poll_fn, ts, PDCH_ULC_POLL_DL_ACK);
 
 			m_tx_counter = 0;
 			/* start timer whenever we send the final block */