[PATCH] The redefinition of ahci_start_engine() and ahci_stop_engine()

- Make ahci_start_engine() and ahci_stop_engine() more consistent with
  AHCI spec 1.1
- Change their input parameter from ap to port_mmio
- Update the existing users of ahci_start_engine() and ahci_stop_engine()

Signed-off-by: Forrest Zhao <forrest.zhao@intel.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>