IB/mlx5: Support default partition key for representor port

Representor port has only one default pkey.  Hence have simpler query pkey
callback or it.

Link: https://lore.kernel.org/r/20210127150010.1876121-4-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>