[PATCH] libata: implement ata_port_max_devices()

Implement ata_port_max_devices().  This function returns the number of
possible devices on a port.  This will be used by new PM
implementation.

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