ide: use ide_complete_cmd() for REQ_UNPARK_HEADS

* Fixup ->tf_flags in ide_do_park_unpark() to match their current use.

* Use ide_complete_cmd() for REQ_UNPARK_HEADS.

While at it:

* No need to read Error register for PM requests in task_no_data_intr().

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