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_data.cpp b/src/gprs_rlcmac_data.cpp
index 06be8b7..af320c7 100644
--- a/src/gprs_rlcmac_data.cpp
+++ b/src/gprs_rlcmac_data.cpp
@@ -955,9 +955,9 @@
 }
 
 struct msgb *gprs_rlcmac_send_packet_uplink_assignment(
+	struct gprs_rlcmac_bts *bts,
 	struct gprs_rlcmac_tbf *tbf, uint32_t fn)
 {
-	struct gprs_rlcmac_bts *bts = gprs_rlcmac_bts;
 	struct msgb *msg;
 	struct gprs_rlcmac_tbf *new_tbf;