thunderbolt: Fix a leak in tb_retimer_add()

After the device_register() succeeds, then the correct way to clean up
is to call device_unregister().  The unregister calls both device_del()
and device_put().  Since this code was only device_del() it results in
a memory leak.

Fixes: dacb1287 ("thunderbolt: Add support for on-board retimers")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>