tbf/bts: Rename tbf->snd_dl_ack to tbf->rcvd_dl_ack

This function is called to act upon a received DL ACK packet so this
name makes more sense.
diff --git a/src/bts.cpp b/src/bts.cpp
index ff16e29..6c02408 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -805,7 +805,7 @@
 	LOGP(DRLCMAC, LOGL_DEBUG, "RX: [PCU <- BTS] %s Packet Downlink Ack/Nack\n", tbf_name(tbf));
 	tbf->poll_state = GPRS_RLCMAC_POLL_NONE;
 
-	rc = tbf->snd_dl_ack(
+	rc = tbf->rcvd_dl_ack(
 		ack_nack->Ack_Nack_Description.FINAL_ACK_INDICATION,
 		ack_nack->Ack_Nack_Description.STARTING_SEQUENCE_NUMBER,
 		ack_nack->Ack_Nack_Description.RECEIVED_BLOCK_BITMAP);