IB/mlx5: Check the correct variable in error handling code

The code accidentally checks "event_sub" instead of "event_sub->eventfd".

Fixes: 75973853 ("IB/mlx5: Enable subscription for device events over DEVX")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20190807123236.GA11452@mwanda


Signed-off-by: Doug Ledford <dledford@redhat.com>