RDMA/uverbs: Refactor rdma_counter_set_auto_mode and __counter_set_mode

Success is returned in the following flows:
 * New mode is the same as the current one.
 * Switched to new mode and there are no bound counters yet.

Link: https://lore.kernel.org/r/20210318110502.673676-1-leon@kernel.org


Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Reviewed-by: Mark Zhang <markzhang@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>