net: hns3: add protection when get SFP speed as 0

In some case, the MAC speed get from hardware maybe 0, it should
not be set to mac->speed.

Signed-off-by: Guojia Liao <liaoguojia@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>