net: hns3: add MTU initialization for hardware

When initializing the MAC, the MTU vlaue need to be set to the hardware
too. Otherwise, the MTU value of software will be different from the MTU
value of hardware.

Fixes: 46a3df9f

 ("net: hns3: Add HNS3 Acceleration Engine & Compatibility Layer Support")
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>