bts: Remove global state from gprs_rlcmac_send_packet_uplink_assignment

This was the last method of gprs_rlcmac_data.cpp to access the
gprs_rlcmac_bts variable.
diff --git a/src/gprs_rlcmac_sched.cpp b/src/gprs_rlcmac_sched.cpp
index 2dfc486..82e587a 100644
--- a/src/gprs_rlcmac_sched.cpp
+++ b/src/gprs_rlcmac_sched.cpp
@@ -134,7 +134,7 @@
 	/* schedule PACKET UPLINK ASSIGNMENT (1st priority) */
 	if (ul_ass_tbf) {
 		tbf = ul_ass_tbf;
-		msg = gprs_rlcmac_send_packet_uplink_assignment(tbf, fn);
+		msg = gprs_rlcmac_send_packet_uplink_assignment(bts, tbf, fn);
 	}
 	/* schedule PACKET DOWNLINK ASSIGNMENT (2nd priotiry) */
 	if (!msg && dl_ass_tbf) {