ide: factor out devices init from ide_init_port_data()

* Factor out devices init from ide_init_port_data() to
  ide_port_init_devices_data().

While at it:

* Add explicit clearing of IDE device structure.

There should be no functionality changes caused by this patch.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>