[PATCH] libata: modify ata_dev_try_classify

Make ata_dev_try_classify take @r_err to store tf error register value
on completion and return device class instead of directly manipulating
dev->class.  This is preparation for new reset mechanism.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>