ide: Remove ide_spin_wait_hwgroup() and use special requests instead

Use a special request for serialisation purposes and get rid of the
awkward ide_spin_wait_hwgroup(). This also involves converting the
ide_devset structure so it can be shared by the /proc and the ioctl code.

Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
[bart: use rq->cmd[] directly]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>