pcu_l1_if: add frame number to log output

Currently, the log output lacks the frame number.

 - make get_current_fn() public
 - add frame number to the log statements in pcu_l1_if.cpp

Change-Id: Idce994dbf86a2bbf861907d75418a2a3867244db
Related: SYS#4139
Patch-by: Octasic inc.
diff --git a/src/gsm_timer.cpp b/src/gsm_timer.cpp
index d3c59cb..f7901d4 100644
--- a/src/gsm_timer.cpp
+++ b/src/gsm_timer.cpp
@@ -43,7 +43,7 @@
  * all time functions schedule based on the BTS they
  * are scheduled on.
  */
-static int get_current_fn()
+int get_current_fn()
 {
 	return BTS::main_bts()->current_frame_number();
 }