net/mlx5e: Fix mlx5e_tx_reporter_create return value

Return error when failing to create a reporter in devlink. Since
NET_DEVLINK mandatory to MLX5_CORE in Kconfig, returned pointer
can't be NULL and can only hold an error in bad path.

Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>