net: hns: fix bug that alloc skb fail lead to port unavailable

When hns_nic_poll_rx_skb alloc skb fail, it will break receive cycle and
read new fbd_num to start new receive cycle. It recomputes cycle num is
fbd_num minus clean_count, actually this cycle num is too big because
it drop out receive cycle. It brings about the port unavailable.

So we will goto out when alloc skb fail to fix this bug.

Signed-off-by: Jun He <hjat2005@huawei.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>