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/tbf_dl.cpp b/src/tbf_dl.cpp
index 856dae6..0e6b922 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -1027,7 +1027,7 @@
 	m_dl_ack_requested = true;
 }
 
-void tbf_dl_request_dl_ack(struct gprs_rlcmac_dl_tbf *dl_tbf) {
+void dl_tbf_request_dl_ack(struct gprs_rlcmac_dl_tbf *dl_tbf) {
 	dl_tbf->request_dl_ack();
 }