sata_via: magic vt6421 fix for transmission problems w/ WD drives

vt6421 has problems talking to recent WD drives. It causes a lot of
transmission errors while high bandwidth transfer as reported in the
following bugzilla entry.

https://bugzilla.kernel.org/show_bug.cgi?id=15173

Joseph Chan provided the following fix. I don't have any idea what it
does but I can verify the issue is gone with the patch applied.

Signed-off-by: Tejun Heo <[e-mail]>
Originally-from: Joseph Chan <[e-mail]>
Reported-by: Jorrit Tijben <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jeff Garzik <[e-mail]>

drivers/ata/sata_via.c

···
 		tmp8 |= NATIVE_MODE_ALL;
 		pci_write_config_byte(pdev, SATA_NATIVE_MODE, tmp8);
 	}
+
+	/*
+	 * vt6421 has problems talking to some drives.  The following
+	 * is the magic fix from Joseph Chan <[e-mail]>.
+	 * Please add proper documentation if possible.
+	 *
+	 * https://bugzilla.kernel.org/show_bug.cgi?id=15173
+	 */
+	if (pdev->device == 0x3249) {
+		pci_read_config_byte(pdev, 0x52, &tmp8);
+		tmp8 |= 1 << 2;
+		pci_write_config_byte(pdev, 0x52, tmp8);
+	}
 }
 
 static int svia_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)