tty: serial: pch: don't crash if DMA enabled but not loaded

if the DMA driver isn't loaded "on time" then we crash in the irq handler:
| pch_uart 0000:02:0a.4: pch_request_dma:dma_request_channel FAILS(Tx)
| BUG: unable to handle kernel NULL pointer dereference at   (null)
| IP: [<c0676ed9>] pch_uart_interrupt+0x739/0x940

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/pch_uart.c

@@ -1508,10 +1508,14 @@ static int pch_uart_verify_port(struct uart_port *port,
 			__func__);
 		return -EOPNOTSUPP;
 #endif
-		dev_info(priv->port.dev, "PCH UART : Use DMA Mode\n");
-		if (!priv->use_dma)
+		if (!priv->use_dma) {
 			pch_request_dma(port);
-		priv->use_dma = 1;
+			if (priv->chan_rx)
+				priv->use_dma = 1;
+		}
+		dev_info(priv->port.dev, "PCH UART: %s\n",
+				priv->use_dma ?
+				"Use DMA Mode" : "No DMA");
 	}
 
 	return 0;