[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...

openbsc/src/abis_nm.c

@@ -1581,6 +1581,7 @@ static int abis_nm_rcvmsg_sw(struct msgb *mb)
 				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) {