Move WAIT_RELEASE tbf_state transition to tbf_fsm

While at it, method maybe_start_new_window is renamed to
rcvd_dl_final_ack to make more sense out of the code.

Related: OS#2709
Change-Id: Iebd650c1036ef2d5132789778be7117ce3391c01
diff --git a/src/tbf_dl.h b/src/tbf_dl.h
index 58d863b..af82a0c 100644
--- a/src/tbf_dl.h
+++ b/src/tbf_dl.h
@@ -110,7 +110,7 @@
 					int index, int index2 = -1);
 	int update_window(const uint8_t ssn, const uint8_t *rbb);
 	int update_window(unsigned first_bsn, const struct bitvec *rbb);
-	int maybe_start_new_window();
+	int rcvd_dl_final_ack();
 	bool dl_window_stalled() const;
 	void reuse_tbf();
 	void start_llc_timer();