pata_cmd64x: fix overclocking of UDMA0-2 modes

adev->dma_mode stores the transfer mode value not UDMA mode number
so the condition in cmd64x_set_dmamode() is always true and the higher
UDMA clock is always selected.  This can potentially result in data
corruption when UDMA33 device is used, when 40-wire cable is used or
when the error recovery code decides to lower the device speed down.

The issue was introduced in the commit 6a40da02 ("libata cmd64x: whack
into a shape that looks like the documentation") which goes back to
kernel 2.6.20.

Cc: stable@kernel.org
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/pata_cmd64x.c

@@ -219,7 +219,7 @@ static void cmd64x_set_dmamode(struct ata_port *ap, struct ata_device *adev)
 		regU |= udma_data[adev->dma_mode - XFER_UDMA_0] << shift;
 		/* Merge the control bits */
 		regU |= 1 << adev->devno; /* UDMA on */
-		if (adev->dma_mode > 2)	/* 15nS timing */
+		if (adev->dma_mode > XFER_UDMA_2) /* 15nS timing */
 			regU |= 4 << adev->devno;
 	} else {
 		regU &= ~ (1 << adev->devno);	/* UDMA off */