Cleanup FN scheduling

* replace magic number with defined constant
* move copy-pasted code to inline functions
* remove unused code

Change-Id: I6fee0714453d0c3c3f3f875f88daea2d9c477331
Related: OS#1524
diff --git a/src/gprs_rlcmac_sched.cpp b/src/gprs_rlcmac_sched.cpp
index 97ee53e..a21c023 100644
--- a/src/gprs_rlcmac_sched.cpp
+++ b/src/gprs_rlcmac_sched.cpp
@@ -41,7 +41,7 @@
 	poll_fn = fn + 4;
 	if ((block_nr % 3) == 2)
 		poll_fn ++;
-	poll_fn = poll_fn % 2715648;
+	poll_fn = poll_fn % GSM_MAX_FN;
 	llist_for_each(pos, &bts->ul_tbfs()) {
 		ul_tbf = as_ul_tbf(pos->entry());
 		OSMO_ASSERT(ul_tbf);