Make probe_hwif() return an error value.

While at it:

* Remove comment about MAX_DRIVES == 2 limitation (it is not special to
  probe_hwif(), it is a general assumption taken by a lot of IDE code).

* Rename probe_hwif() to ide_probe_port().
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>