ide: add ide_execute_pkt_cmd() helper

Add ide_execute_pkt_cmd() helper for executing PACKET command,
then convert ATAPI device drivers to use it.

As a nice side-effect this fixes ide-{floppy,tape,scsi} w.r.t.
ide_lock taking (ide-cd was OK).

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