devlink: remove "gen" from struct devlink_gen_cmd name

No need to have "gen" inside name of the structure for devlink commands.
Remove it.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>