net: hns3: Add configure for mac minimal frame size

When change the mtu, the minimal frame size of mac will be set
to zero, it is incorrect. This patch fixes it by set it to the
default value.

Signed-off-by: Jian Shen <shenjian15@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>