mgcp: Change API to remove memory management from the name

Jacob pointed out that "free_endp" refers to the memory of
the endpoint being freed. What we want is actually a way to
release an endpoint (and the resource it allocated) or in
the case of the testcase/testapp initialize the data structure
correctly. Introduce two names for that.
diff --git a/openbsc/tests/mgcp/mgcp_transcoding_test.c b/openbsc/tests/mgcp/mgcp_transcoding_test.c
index 2e857a1..44f3072 100644
--- a/openbsc/tests/mgcp/mgcp_transcoding_test.c
+++ b/openbsc/tests/mgcp/mgcp_transcoding_test.c
@@ -180,7 +180,7 @@
 	tcfg->cfg = cfg;
 	endp->tcfg = tcfg;
 	endp->cfg = cfg;
-	mgcp_free_endp(endp);
+	mgcp_initialize_endp(endp);
 
 	dst_end = &endp->bts_end;
 	dst_end->payload_type = audio_name_to_type(dstfmt);