net: hns3: move struct hclge_mdio_cfg_cmd declaration

driver inclusion
category: cleanup
bugzilla: NA
CVE: NA

As struct hclge_mdio_cfg_cmd is only used in hclge_mdio.c, this patch
move its declaration from hclge_mdio.h to hclge_mdio.c.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Reviewed-by: Zhong Zhaohui <zhongzhaohui@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>