net/mlx5e: Statify function mlx5_cmd_trigger_completions

Starting from commit
4cab346b

 ("net/mlx5: No command allowed when command interface is not ready"),
no calls to mlx5_cmd_trigger_completions() are external to cmd.c anymore.
Make it a static function.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>