RDMA/hns: Validate the pkey index

Before query pkey, make sure that the queried index is valid.

Fixes: 9a443537

 ("IB/hns: Add driver files for hns RoCE driver")
Link: https://lore.kernel.org/r/20211117145954.123893-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>