libata: separate out ata_dev_reread_id()

Separate out ata_dev_reread_id() from ata_dev_revalidate().
ata_dev_reread_id() reads IDENTIFY page and determines whether the
same device is still there.  ata_dev_revalidate() reconfigures after
reread completes.  This will be used by ACPI update.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>