RDMA/nldev: Prevent underflow in nldev_stat_set_counter_dynamic_doit()

This code checks "index" for an upper bound but it does not check for
negatives.  Change the type to unsigned to prevent underflows.

Fixes: 3c3c1f14

 ("RDMA/nldev: Allow optional-counter status configuration through RDMA netlink")
Link: https://lore.kernel.org/r/20220316083948.GC30941@kili
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>