udevadm: normalize exit status

After d354690e7d, each udevadm command
returns negative errno on error. So, let's normalize the exit status
of udevadm.

src/udev/udevadm.c

@@ -124,7 +124,5 @@ finish:
         mac_selinux_finish();
         log_close();
 
-        /* TODO: fix this later.
-         * Currently, verbs return positive values on failure. */
-        return r < 0 ? EXIT_FAILURE : r;
+        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
 }