ide: use per-device request queue locks (v2)

ide: use per-device request queue locks (v2)

* Move hack for flush requests from choose_drive() to do_ide_request().

* Add ide_plug_device() helper and convert core IDE code from using
  per-hwgroup lock as a request lock to use the ->queue_lock instead.

* Remove no longer needed:
  - choose_drive() function
  - WAKEUP() macro
  - 'sleeping' flag from ide_hwif_t
  - 'service_{start,time}' fields from ide_drive_t

This patch results in much simpler and more maintainable code
(besides being a scalability improvement).

v2:
* Fixes/improvements based on review from Elias:
  - take as many requests off the queue as possible
  - remove now redundant BUG_ON()

Cc: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>