[ide] add ide_hwif_t->dma_exec_cmd()

[ide] add ide_hwif_t->dma_exec_cmd()

* split off ->dma_exec_cmd() from ->ide_dma_[read,write] functions
* choose command to execute by ->dma_exec_cmd() in higher layers
  and remove ->ide_dma_[read,write]
* in Etrax ide.c driver REQ_DRIVE_TASKFILE requests weren't
  handled properly for drive->addressing == 0
* in trm290.c read and write commands were interchanged
* in sgiioc4.c commands weren't sent to disk devices

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