net: hns3: optimize parameter of hclge_set_phy_loopback() function

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

This patch deletes the parameter phydev of hclge_set_phy_loopback()
function for optimization, as it can be gotten by hdev->hw.mac.phydev.

The variable phydev should be checked whether null pointer before use.

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