net/mlx5: potential error pointer dereference in error handling

The error handling was a bit flipped around.  If the mlx5_create_flow_group()
function failed then it would have resulted in dereferencing "fg" when
it was an error pointer.

Fixes: 80f09dfc

 ("net/mlx5: Eswitch, enable RoCE loopback traffic")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>