net: hns3: add support for obtaining the maximum frame size

Since the newer hardware may supports different frame size,
so add support to obtain the capability from the firmware
instead of the fixed value.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>