ide: move hwif_register() call out of ide_probe_port()

* Add BUG_ON(hwif->present) at the start of ide_probe_port().

* Move hwif_register() call (along with setting hwif->present) from
  ide_probe_port() to ide_device_add_all().

  As a result the port will be registered with the device tree _after_:
  - probing both devices (if both are present)
  - port reset (if hwif->reset is set)
  - restoring local IRQs state and re-enabling port IRQ

While at it:

* Rename hwif_register() to ide_register_port().

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>