[media] dvb_frontend: print a msg if a property doesn't exist

If userspace calls a property that doesn't exist, it currently
just returns -EINVAL. However, this is more likely a problem at
the userspace application, calling it with a non-existing property.
So, add a debug message to help tracking it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/dvb-core/dvb_frontend.c

@@ -1479,6 +1479,9 @@ static int dtv_property_process_get(struct dvb_frontend *fe,
 		tvp->u.st = c->block_count;
 		break;
 	default:
+		dev_dbg(fe->dvb->device,
+			"%s: FE property %d doesn't exist\n",
+			__func__, tvp->cmd);
 		return -EINVAL;
 	}