use enum for operational state
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index 98683d3..c99ffcf 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -540,11 +540,11 @@
 const char *nm_opstate_name(u_int8_t os)
 {
 	switch (os) {
-	case 1:
+	case NM_OPSTATE_DISABLED:
 		return "Disabled";
-	case 2:
+	case NM_OPSTATE_ENABLED:
 		return "Enabled";
-	case 0xff:
+	case NM_OPSTATE_NULL:
 		return "NULL";
 	default:
 		return "RFU";