Encapsulate handling of UL ACK timeout

Use helper methods instead checking and manipulating flag directly.

Change-Id: Ia3f009c52118db95b38a077e08eecda844e7f8d1
Related: OS#1539
diff --git a/src/tbf.h b/src/tbf.h
index 87aa7a5..ecdf5f8 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -503,6 +503,8 @@
 	gprs_rlcmac_ul_tbf(BTS *bts);
 
 	struct msgb *create_ul_ack(uint32_t fn, uint8_t ts);
+	bool ctrl_ack_to_toggle();
+	bool handle_ctrl_ack();
 
 	/* blocks were acked */
 	int rcv_data_block_acknowledged(