net/mlx5e: Fix mlx5e_tx_reporter_create return value

If reporter is ERR_PTR or NULL, error code shall be returned. At all other
cases it shall return success. Fix that.

Fixes: de8650a8 ("net/mlx5e: Add tx reporter support")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>