[media] mantis_dvb: fix some error codes in mantis_dvb_init()

We should be returning negative error codes here or it leads to a crash.
This also silences a static checker warning.

	drivers/media/pci/mantis/mantis_cards.c:250 mantis_pci_probe()
	warn: 'mantis->dmxdev.dvbdev->fops' double freed
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/mantis/mantis_dvb.c

@@ -226,11 +226,12 @@ int mantis_dvb_init(struct mantis_pci *mantis)
 			goto err5;
 		} else {
 			if (mantis->fe == NULL) {
+				result = -ENOMEM;
 				dprintk(MANTIS_ERROR, 1, "FE <NULL>");
 				goto err5;
 			}
-
-			if (dvb_register_frontend(&mantis->dvb_adapter, mantis->fe)) {
+			result = dvb_register_frontend(&mantis->dvb_adapter, mantis->fe);
+			if (result) {
 				dprintk(MANTIS_ERROR, 1, "ERROR: Frontend registration failed");
 
 				if (mantis->fe->ops.release)