net: hns3: prevent change MTU when resetting

When resetting, the changing of MTU is not allowed, so this patch
adds checking reset status in hns3_nic_change_mtu() to do that.

Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>