[PATCH] libata-core cleanups (updated)

libata-core cleanups:
- use kzalloc() instead of kmalloc() + memset();
- use one exit path in ata_device_add();

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>