drivers/rpmsg: Use tx buffer size as payload length

Directly use tx length received from hub as the tx payload length.

Signed-off-by: yintao <yintao@xiaomi.com>

drivers/rpmsg/rpmsg_router_edge.c

@@ -159,7 +159,7 @@ rpmsg_router_edge_get_tx_payload_buffer(FAR struct rpmsg_device *rdev,
     }
 
   buf = hubdev->ops.get_tx_payload_buffer(hubdev, len, wait);
-  *len = edge->tx_len - (rpmsg_get_tx_buffer_size(rdev) - *len);
+  *len = edge->tx_len;
   return buf;
 }