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>

diff --git a/drivers/ide/pci/sc1200.c b/drivers/ide/pci/sc1200.c
index 8d762d3..08e317f 100644
--- a/drivers/ide/pci/sc1200.c
+++ b/drivers/ide/pci/sc1200.c
@@ -241,10 +241,7 @@
 
 	outb(inb(hwif->dma_base+2)|(unit?0x40:0x20), hwif->dma_base+2);	/* set DMA_capable bit */
 
-	/*
-	 * Finally, turn DMA on in software, and exit.
-	 */
-	return hwif->ide_dma_on(drive);	/* success */
+	return 0;	/* success */
 }
 
 /*