bts: Move the frame_number into the BTS sructure

The current_frame is an attribute of the BTS. Move it from the
pcu_l1_if.cpp into the BTS. As the next step we can trigger
actions depending on the change of the frame.
diff --git a/src/bts.cpp b/src/bts.cpp
index 0609a5f..b278ecf 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -39,7 +39,13 @@
 }
 
 BTS::BTS()
+	: m_cur_fn(0)
 {
 	memset(&m_bts, 0, sizeof(m_bts));
 	m_bts.bts = this;
 }
+
+void BTS::set_current_frame_number(int fn)
+{
+	m_cur_fn = fn;
+}