net: hns3: refine the definition for struct hclge_pf_to_vf_msg

The struct hclge_pf_to_vf_msg is used for mailbox message from
PF to VF, including both response and request. But its definition
can only indicate respone, which makes the message data copy in
function hclge_send_mbx_msg() unreadable. So refine it by edding
a general message definition into it.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>