RDMA/ocrdma: Simplify process_mad function

Change the switch with one case into a simple if statement so the code is
less confusing.

Link: https://lore.kernel.org/r/20191029062745.7932-12-leon@kernel.org


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