gsm_data_shared: introduce 'struct gsm_abis_mo'

... as a common wrapper around nm_attr and nm_state
diff --git a/openbsc/src/libcommon/gsm_data.c b/openbsc/src/libcommon/gsm_data.c
index 54e7a63..b1fb4db 100644
--- a/openbsc/src/libcommon/gsm_data.c
+++ b/openbsc/src/libcommon/gsm_data.c
@@ -147,7 +147,7 @@
 
 	trx->bts = bts;
 	trx->nr = bts->num_trx++;
-	trx->nm_state.administrative = NM_STATE_UNLOCKED;
+	trx->mo.nm_state.administrative = NM_STATE_UNLOCKED;
 
 	for (k = 0; k < TRX_NR_TS; k++) {
 		struct gsm_bts_trx_ts *ts = &trx->ts[k];