[AGPGART] Silence serverworks CNB20HE printks.

These chipsets don't seem to have a full AGP GART, (and in at least
two reports I've had, are used in systems that don't even have an
AGP slot).
Print something more informative than..
                                                                                           
  agpgart: unable to determine aperture size.
  agpgart: agp_backend_initialize() failed.
  agpgart-serverworks: probe of 0000:00:00.0 failed with error -22
  agpgart: Maximum main memory to use for agp memory: 941M
  agpgart: unable to determine aperture size.
  agpgart: agp_backend_initialize() failed.
  agpgart-serverworks: probe of 0000:00:00.1 failed with error -22
  agpgart: Unsupported Serverworks chipset (device id: 0006)
  agpgart: Unsupported Serverworks chipset (device id: 0006)

drivers/char/agp/sworks-agp.c

@@ -454,10 +454,17 @@ static int __devinit agp_serverworks_probe(struct pci_dev *pdev,
 	}
 
 	switch (pdev->device) {
+	case 0x0006:
+		/* ServerWorks CNB20HE
+		Fail silently.*/
+		printk (KERN_ERR PFX "Detected ServerWorks CNB20HE chipset: No AGP present.\n");
+		return -ENODEV;
+
 	case PCI_DEVICE_ID_SERVERWORKS_HE:
 	case PCI_DEVICE_ID_SERVERWORKS_LE:
 	case 0x0007:
 		break;
+
 	default:
 		printk(KERN_ERR PFX "Unsupported Serverworks chipset "
 				"(device id: %04x)\n", pdev->device);