libata-link: separate out ata_eh_handle_dev_fail()

Separate out ata_eh_handle_dev_fail() from ata_eh_recover().  This is
in preparation of ata_link and PMP support.

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