RDMA/mlx5: Free IB device on remove

Simplify the code and move the deallocation of the IB device into the
remove function.

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