pata_imx: cleanup error path

- rename free_priv label to 'err' since priv is allocated with devm_*
  and not freed here.
- add missing 'goto err' in case ata_host_activate fails
- add 'ret' variable to return correct error value instead of hardcoded
  -ENOMEM in error case.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>