net: hns3: fix for changing MTU

when changing MTU, The new MTU must need to be set to netdevice.

Fixes: a8e8b7ff ("net: hns3: Add support to change MTU in HNS3 hardware")
Signed-off-by: Fuyun Liang <liangfuyun1@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>