ide: add ide_end_rq() (v2)

* Move request dequeuing from __ide_end_request() to ide_end_request().

* Rename __ide_end_request() to ide_end_rq() and export it.

* Fix ide_end_rq() to pass original blk_end_request() return value.

* ide_end_dequeued_request() is used only in cdrom_end_request()
  so inline it there and then remove the function.

v2:
* Remove needless BUG_ON() while at it (start_request()'s one is enough).

There should be no functional changes caused by this patch.

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