pdch: Move handling of control_ack to a separate method

Kill the unused tfi parameter
diff --git a/src/bts.h b/src/bts.h
index 50eb434..b155b5c 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -81,6 +81,7 @@
 	int rcv_data_block_acknowledged(uint8_t *data, uint8_t len, int8_t rssi);
 	int rcv_control_block(bitvec *rlc_block, uint32_t fn);
 
+	void rcv_control_ack(RlcMacUplink_t *, uint32_t fn);
 #endif
 };