ide: add ide_set_dma() helper (v2)

* add ide_set_dma() helper and make ide_hwif_t.ide_dma_check return
  -1 when DMA needs to be disabled (== need to call ->ide_dma_off_quietly)
   0 when DMA needs to be enabled  (== need to call ->ide_dma_on)
   1 when DMA setting shouldn't be changed
* fix IDE code to use ide_set_dma() instead if using ->ide_dma_check directly

v2:
* updated for scc_pata

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

drivers/ide/pci/sgiioc4.c

@@ -296,9 +296,9 @@ static int sgiioc4_ide_dma_check(ide_drive_t *drive)
 	if (ide_config_drive_speed(drive, XFER_MW_DMA_2) != 0) {
 		printk(KERN_WARNING "%s: couldn't set MWDMA2 mode, "
 				    "using PIO instead\n", drive->name);
-		return sgiioc4_ide_dma_off_quietly(drive);
+		return -1;
 	} else
-		return sgiioc4_ide_dma_on(drive);
+		return 0;
 }
 
 /* returns 1 if dma irq issued, 0 otherwise */