ide: add pointer to the current packet command to ide_drive_t

ide: add pointer to the current packet command to ide_drive_t

* Add pointer to the current packet command (struct ide_atapi_pc *pc)
  to ide_drive_t and use it instead of the pointer in struct ide_*_obj.

* Use drive->pc in ide_{issue,transfer}_pc() and ide_pc_intr()
  instead of 'pc' argument.

There should be no functional changes caused by this patch.

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