Use -Werror in hardware/libhardware/modules

* Remove unused local variables.
* Remove or comment out unused static functions.
* Fix trivial bugs found by unused value warnings.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I99389b883c89551850180d25241a35a40bb77b26
diff --git a/modules/vehicle/vehicle.c b/modules/vehicle/vehicle.c
index 66e29c0..66a2137 100644
--- a/modules/vehicle/vehicle.c
+++ b/modules/vehicle/vehicle.c
@@ -465,6 +465,9 @@
     }
     int ret_code = pthread_create(
                                   &sub->thread, NULL, (void *(*)(void*))fake_event_thread, sub);
+    if (ret_code != 0) {
+        return -ret_code;
+    }
     print_subscribe_info(impl);
     pthread_mutex_unlock(&lock_);
     return 0;