net: hinic: Set max_mtu/min_mtu directly to simplify the code.

Set max_mtu/min_mtu directly to avoid making the validity judgment
when set mtu, because the judgment is made in net/core: dev_validate_mtu,
so to simplify the code.

Signed-off-by: caihuoqing <cai.huoqing@linux.dev>
Link: https://lore.kernel.org/r/20221024103349.4494-1-cai.huoqing@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>