ide: convert do_rw_taskfile() to use ->data_phase

* Use task->data_phase in do_rw_taskfile() to decide what to do.

* task->prehandler is only used by TASKFILE[_MULTI]_OUT so just
  use pre_task_out_intr() directly and remove no longer needed
  'prehandler' field from ide_task_t.

* Remove no longer needed ide_pre_handler_t type.

There should be no functionality changes caused by this patch.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>