posix: sockets: Fix return value of listen function

If an error happens, listen was returning 1 instead of -1.

Signed-off-by: Francois Berder <francois.berder@imgtec.com>

sys/posix/sockets/posix_sockets.c

@@ -752,7 +752,7 @@ int listen(int socket, int backlog)
         bf_unset(_sock_pool_used, _get_sock_idx(sock));
         mutex_unlock(&_socket_pool_mutex);
     }
-    return -res;
+    return res;
 #else
     (void)socket;
     (void)backlog;