libata: Support PIO polling-only hosts.

By default ata_host_activate() expects a valid IRQ in order to
successfully register the host. This patch enables a special case
for registering polling-only hosts that either don't have IRQs
or have buggy IRQ generation (either in terms of handling or
sensing), which otherwise work fine.

Hosts that want to use polling mode can simply set ATA_FLAG_PIO_POLLING
and pass in an invalid IRQ.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>