devlink: rename and reorder instances of struct devlink_cmd

In order to maintain naming consistency, rename and reorder all usages
of struct struct devlink_cmd in the following way:
1) Remove "gen" and replace it with "cmd" to match the struct name
2) Order devl_cmds[] and the header file to match the order
   of enum devlink_command
3) Move devl_cmd_rate_get among the peers
4) Remove "inst" for DEVLINK_CMD_GET
5) Add "_get" suffix to all to match DEVLINK_CMD_*_GET (only rate had it
   done correctly)

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>