RDMA/nldev: Return an error message on failure to turn auto mode

The bounded counter can't be reconfigured to be in auto mode, in attempt
to do it, the user will get an error, but without any hint why. Update
nldev interface to return an error message through extack mechanism.

Link: https://lore.kernel.org/r/20201230130240.180737-1-leon@kernel.org
Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>