Don't access TBF internals in vty functions

Obtain corresponding window object of UL/DL TBF by using proper accessor
function instead of direct access to private member.

Change-Id: I89bcd2c2b0b6f120d40d20fd43c1e516de3e3950
diff --git a/src/pcu_vty_functions.cpp b/src/pcu_vty_functions.cpp
index 0a80a23..001164c 100644
--- a/src/pcu_vty_functions.cpp
+++ b/src/pcu_vty_functions.cpp
@@ -67,7 +67,7 @@
 	vty_out(vty, " CS=%s", tbf->current_cs().name());
 
 	if (ul_tbf) {
-		gprs_rlc_ul_window *win = &ul_tbf->m_window;
+		gprs_rlc_ul_window *win = ul_tbf->window();
 		vty_out(vty, " WS=%u V(Q)=%d V(R)=%d",
 			ul_tbf->window_size(), win->v_q(), win->v_r());
 		vty_out(vty, "%s", VTY_NEWLINE);
@@ -79,7 +79,7 @@
 		}
 	}
 	if (dl_tbf) {
-		gprs_rlc_dl_window *win = &dl_tbf->m_window;
+		gprs_rlc_dl_window *win = dl_tbf->window();
 		vty_out(vty, " WS=%u V(A)=%d V(S)=%d nBSN=%d%s",
 			dl_tbf->window_size(), win->v_a(), win->v_s(), win->resend_needed(),
 			win->window_stalled() ? " STALLED" : "");