net: hns3: refactor the mailbox message between PF and VF

For making the code more readable, this adds several new
structure to replace the msg field in structure
hclge_mbx_vf_to_pf_cmd and hclge_mbx_pf_to_vf_cmd.
Also uses macro to instead of some magic number.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>