ide-disk: add ide_tf_set_cmd() helper

* Add ide_tf_set_cmd() helper for selecting/setting command and data phase
  (note: DMA data phases are there for completness, they are not required ATM).

* Set IDE_TFLAG_WRITE taskfile flag for write requests in __ide_do_rw_disk().

* Convert __ide_do_rw_disk() to use the new ide_tf_set_cmd() helper.

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>