TBF: make poll state internal

* add functions/macros for setting TBF's poll state
* add function for checking TBF's poll state

Change-Id: I6db1c4e7bd0a49aeb5e391afe371c36b96c6a702
Related: OS#1539
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index fdbbd16..80e3831 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -635,7 +635,7 @@
 	/* reset N3105 */
 	n3105 = 0;
 	t_stop(T3191, "ACK/NACK received");
-	poll_state = GPRS_RLCMAC_POLL_NONE;
+	TBF_POLL_SCHED_UNSET(this);
 
 	return ack_recovered;
 }
@@ -1181,7 +1181,7 @@
 
 bool gprs_rlcmac_dl_tbf::need_control_ts() const
 {
-	if (poll_state != GPRS_RLCMAC_POLL_NONE)
+	if (poll_scheduled())
 		return false;
 
 	return state_flags & (1 << GPRS_RLCMAC_FLAG_TO_DL_ACK) ||