tbf: Move gprs_rlcmac_send_packet_downlink_assignment into the tbf

This method was called _send_packet_ but actually didn't do any
sending at all. It just formated the right assignment. I replaced
tbf-> with nothing.
diff --git a/src/tbf.h b/src/tbf.h
index 4afc649..598bb25 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -99,6 +99,8 @@
 	/* TODO: extract LLC class? */
 	int assemble_forward_llc(uint8_t *data, uint8_t len);
 
+	struct msgb *create_dl_ass(uint32_t fn);
+
 	int rlcmac_diag();
 
 	int update();