libata: separate out ata_host_start()

Separate out ata_host_start() from ata_device_add().  ata_host_start()
calls ->port_start on each port if available and freezes the port.

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