net: hns3: up/down netdev in hclge module when setting mtu

Currently netdev is down in enet module, and it is before
mtu range checking in hclge module, which may be cause
netdev being down unnecessarily.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>