RDMA/mlx4: Read pkey table length instead of hardcoded value

RDMA/mlx4: Read pkey table length instead of hardcoded value

If the pkey_table is not available (which is the case when RoCE is not
supported), the cited commit caused a regression where mlx4_devices
without RoCE are not created.

Fix this by returning a pkey table length of zero in procedure
eth_link_query_port() if the pkey-table length reported by the device is
zero.

Link: https://lore.kernel.org/r/20200824110229.1094376-1-leon@kernel.org
Cc: <stable@vger.kernel.org>
Fixes: 1901b91f9982 ("IB/core: Fix potential NULL pointer dereference in pkey cache")
Fixes: fa417f7b520e ("IB/mlx4: Add support for IBoE")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>