RDMA/core: Introduce and use API to read port immutable data

Currently mlx5 driver caches port GID table length for 2 ports.  It is
also cached by IB core as port immutable data.

When mlx5 representor ports are present, which are usually more than 2,
invalid access to port_caps array can happen while validating the GID
table length which is only for 2 ports.

To avoid this, take help of the IB cores port immutable data by exposing
an API to read the port immutable fields.

Remove mlx5 driver's internal cache, thereby reduce code and data.

Link: https://lore.kernel.org/r/20210203130133.4057329-5-leon@kernel.org


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