net/mlx5: remove redundant ret variable

Return value from mlx5dr_send_postsend_action() directly instead of taking
this in another redundant variable.

Signed-off-by: zhang songyi <zhang.songyi@zte.com.cn>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>