ide-scsi: use pc->callback

* Add ide_scsi_callback() pc->callback implementation, then update
  idescsi_check_condition() and idescsi_queue() to setup ->callback.

* Convert idescsi_pc_intr() to use pc->callback.

This is a preparation for adding generic ide_pc_intr() helper.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>