ide: use __generic_unplug_device() in ide_do_drive_cmd() (take 2)

ide: use __generic_unplug_device() in ide_do_drive_cmd() (take 2)

* Call __elv_add_request() with 'plug' == 1 (so the device will be
  plugged) and then use __generic_unplug_device() instead of calling
  ide_do_request() directly.

v2:
* For blk_pm_resume_request() requests the queue is stopped so we
  need to call ->request_fn explicitly.

  Thanks to:
  - Rafael for reporting/bisecting the bug
  - Borislav/Rafael for testing the fix

This is a preparation for converting IDE to use blk_execute_rq().

Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Borislav Petkov <petkovbb@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>