RDMA/mlx5: Simplify multiple else-if cases with switch keyword

Improve readability of fs.c by converting multiple else-if constructions
to be implemented with switch keyword.

Link: https://lore.kernel.org/r/20200730081235.1581127-2-leon@kernel.org


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