bts: Remove global state and the usage of the gprs_rlcmac_bts

Global state makes mocking/writing tests more difficult. Continue
by removing direct usage of the bts and adding it as pointer to
the function calls (showing the true dependencies of the functions)
diff --git a/src/gprs_rlcmac_data.cpp b/src/gprs_rlcmac_data.cpp
index 079af56..c2ed9e6 100644
--- a/src/gprs_rlcmac_data.cpp
+++ b/src/gprs_rlcmac_data.cpp
@@ -1060,7 +1060,7 @@
 	if (qta > 252)
 		qta = 252;
 	if (sb) {
-		rc = sba_alloc(&trx, &ts, &sb_fn, qta >> 2);
+		rc = sba_alloc(bts, &trx, &ts, &sb_fn, qta >> 2);
 		if (rc < 0)
 			return rc;
 		LOGP(DRLCMAC, LOGL_DEBUG, "RX: [PCU <- BTS] RACH qbit-ta=%d "