Rename function tbf_dl_request_dl_ack() -> dl_tbf_request_dl_ack()

Use the dl_tbf prefix which is usually used to easily distinguish from
"ul_tbf" specific APIs and "tbf" generic (parent class) APIs.

Change-Id: Ibf6ae20da99866af5f2b6e12184f3145d1fc0bbf
diff --git a/src/gprs_ms.c b/src/gprs_ms.c
index c072b0a..37071e4 100644
--- a/src/gprs_ms.c
+++ b/src/gprs_ms.c
@@ -99,7 +99,7 @@
 
 	LOGPTBFDL(dl_tbf, LOGL_DEBUG, "LLC receive timeout, requesting DL ACK\n");
 
-	tbf_dl_request_dl_ack(dl_tbf);
+	dl_tbf_request_dl_ack(dl_tbf);
 }
 
 static int ms_talloc_destructor(struct GprsMs *ms);