tbf_dl: Clarify requirements for DL ACK/NACK

Method is renamed since it clearly relates to getting DL ACK/NACK, no
CTRL ACK.

use same methods in both scheduler and internal use since they are
expectd to be run in the same code path by the scheduler. This way we
make sure the same conditions apply and it's clearer when looking at
the code.

Change-Id: Ib0e9b9547f5292b95064bab2dc182fdf659f0518
diff --git a/src/tbf_dl.h b/src/tbf_dl.h
index ad1469a..27b6a2c 100644
--- a/src/tbf_dl.h
+++ b/src/tbf_dl.h
@@ -53,7 +53,7 @@
 	void trigger_ass(struct gprs_rlcmac_tbf *old_tbf);
 
 	void request_dl_ack();
-	bool need_control_ts() const;
+	bool need_poll_for_dl_ack_nack() const;
 	bool have_data() const;
 	int frames_since_last_poll(unsigned fn) const;
 	int frames_since_last_drain(unsigned fn) const;