libata: implement ata_host_detach()

Implement ata_host_detach() which calls ata_port_detach() for each
port in the host and export it.  ata_port_detach() is now internal and
thus un-exported.  ata_host_detach() will be used as the 'deregister
from libata layer' function after devres conversion.

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