Fix Ericsson RBS2000 support after libosmo-abis merge

The libosmo-abis merge broke Ericsson RBS support, as it didn't get the
part right where the per-TRX OML sign_link is determined while
transmitting OM2000 messages.

As a result of this fix, we can remove the 'to_trx_oml' parameter to
_abis_nm_sendmsg(), which is a nice cleanup.
diff --git a/openbsc/include/openbsc/abis_nm.h b/openbsc/include/openbsc/abis_nm.h
index f11ac11..ae9b999 100644
--- a/openbsc/include/openbsc/abis_nm.h
+++ b/openbsc/include/openbsc/abis_nm.h
@@ -160,7 +160,7 @@
 
 void abis_nm_clear_queue(struct gsm_bts *bts);
 
-int _abis_nm_sendmsg(struct msgb *msg, int to_trx_oml);
+int _abis_nm_sendmsg(struct msgb *msg);
 
 void abis_nm_queue_send_next(struct gsm_bts *bts);	/* for bs11_config. */