Get rid of singleton gprs_bssgp_pcu_current_bctx()

Access it from existing pointers instead.

Change-Id: I77455da5221090ebea142ecd49d5dba0065bfc5c
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 87c897f..de43349 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -641,7 +641,7 @@
 		return;
 
 	/* dequeue next LLC frame, if any */
-	msg = llc_dequeue(gprs_bssgp_pcu_current_bctx());
+	msg = llc_dequeue(bts->pcu->bssgp.bctx);
 	if (!msg)
 		return;