[abis] Set rc to 0 to avoid running into a warning about unknown states

When we have received the End Ack we are just doing nothing as we
are done. This means rc remains -1 and we will print a warning but
there is no need to have a warning...
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index db95f21..8b3ac9c 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -1581,6 +1581,7 @@
 				sw->cbfn(GSM_HOOK_NM_SWLOAD,
 					 NM_MT_LOAD_END_ACK, mb,
 					 sw->cb_data, NULL);
+			rc = 0;
 			break;
 		case NM_MT_LOAD_END_NACK:
 			if (sw->forced) {