RDMA/mlx5: Rewrite MAD processing logic to be readable

Multiple "if"s and "||" make extension of process_mad() function
as a tedious task, rewrite that function to be more readable.

Link: https://lore.kernel.org/r/20191029062745.7932-14-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>