IB/mlx5: Free mpi in mp_slave mode

ib_add_slave_port() allocates a multiport struct but never frees it.
Don't leak memory, free the allocated mpi struct during driver unload.

Cc: <stable@vger.kernel.org>
Fixes: 32f69e4b ("{net, IB}/mlx5: Manage port association for multiport RoCE")
Link: https://lore.kernel.org/r/20190916064818.19823-3-leon@kernel.org


Signed-off-by: Danit Goldberg <danitg@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>