net: hns3: Add mtu setting support for vf

The patch adds mtu setting support for vf, currently
vf and pf share the same hardware mtu setting. Mtu set
by vf must be less than or equal to pf' mtu, and mtu
set by pf must be greater than or equal to vf' mtu.

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>