define and use enum values for BS11 (or Siemens?) specific OML message types

diff --git a/src/abis_nm.c b/src/abis_nm.c
index cdb6c5c..7298328 100644
--- a/src/abis_nm.c
+++ b/src/abis_nm.c
@@ -426,13 +426,13 @@
 
 int abis_nm_reset_resource(struct gsm_bts *bts)
 {
-	return __simple_cmd(bts, 0x74);
+	return __simple_cmd(bts, NM_MT_BS11_RESET_RESOURCE);
 }
 
 int abis_nm_db_transmission(struct gsm_bts *bts, int begin)
 {
 	if (begin)
-		return __simple_cmd(bts, 0xA3);
+		return __simple_cmd(bts, NM_MT_BS11_BEGIN_DB_TX);
 	else
-		return __simple_cmd(bts, 0xA6);
+		return __simple_cmd(bts, NM_MT_BS11_END_DB_TX);
 }