ide: dynamic allocation of device structures

Allocate device structures dynamically instead of having them embedded
in ide_hwif_t:

* Remove needless zeroing of port structure from ide_init_port_data().

* Add ide_hwif_t.devices[MAX_DRIVES] (table of pointers to the devices).

* Add ide_port_{alloc,free}_devices() helpers and use them respectively
  in ide_{host,free}_alloc().

* Convert all users of ->drives[] to use ->devices[] instead.

While at it:

* Use drive->dn for the slave device check in scc_pata.c.

As a nice side-effect this patch cuts ~1kB (x86-32) from the resulting
code size:

   text    data     bss     dec     hex filename
  53963    1244     237   55444    d894 drivers/ide/ide-core.o.before
  52981    1244     237   54462    d4be drivers/ide/ide-core.o.after

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