net: hns3: refactor function hclge_mbx_handler()

Currently, the function hclge_mbx_handler() has too many switch-case
statements, it makes this function too long. To improve code readability,
refactor this function and use lookup table instead.

Signed-off-by: Hao Lan <lanhao@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>