gianfar: use netif_tx_queue_stopped instead of __netif_subqueue_stopped

The __netif_subqueue_stopped() just does the following:

        struct netdev_queue *txq = netdev_get_tx_queue(dev, queue_index);
        return netif_tx_queue_stopped(txq);

and since we already have the txq in scope, we can just call that
directly in this case.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>