libata: ensure host is free'd on error exit paths

The host structure is not being kfree'd on two error exit paths
leading to memory leaks. Add in new err_free label and kfree host.

Detected by CoverityScan, CID#1466103 ("Resource leak")

Fixes: 2623c7a5

 ("libata: add refcounting to ata_host")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>