sema: harmonize sema_post return with rest of API

sys/include/sema.h

@@ -133,6 +133,7 @@ static inline int sema_wait(sema_t *sema)
  *
  * @param[in] sema  A semaphore.
  *
+ * @return  0, on success
  * @return  -EINVAL, if semaphore is invalid.
  * @return  -EOVERFLOW, if the semaphore's value would overflow.
  */

sys/sema/sema.c

@@ -169,7 +169,7 @@ int sema_post(sema_t *sema)
         restoreIRQ(old_state);
     }
 
-    return 1;
+    return 0;
 }
 
 /** @} */