ide: fix IDE port slots reservation and freeing (take 2)

* Make ide_find_port_slot() return port slot index.

* Add ide_free_port_slot() helper.

* Move ide_init_port_data() call and error printk() from
  ide_find_port_slot() to ide_host_alloc_all().

* Make ide_{find,free}_port_slot() take ide_cfg_mtx mutex
  and convert them use ide_indexes bitmap to keep track of
  reserved port slots.

* Don't set hwif->chipset in ide_host_alloc_all() as it is
  no longer necessary.

v2:
* Pass the correct slot index to ide_init_port_data().

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