net: hns3: Fix for netdev not up problem when setting mtu

Currently hns3_nic_change_mtu will try to down the netdev before
setting mtu, and it does not up the netdev when the setting fails,
which causes netdev not up problem.

This patch fixes it by not returning when the setting fails.

Fixes: a8e8b7ff

 ("net: hns3: Add support to change MTU in HNS3 hardware")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>