tty: atmel_serial: fix a potential NULL pointer dereference

In case dmaengine_prep_dma_cyclic fails, the fix returns a proper
error code to avoid NULL pointer dereference.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Fixes: 34df42f5 ("serial: at91: add rx dma support")
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/atmel_serial.c

@@ -1288,6 +1288,10 @@ static int atmel_prepare_rx_dma(struct uart_port *port)
 					 sg_dma_len(&atmel_port->sg_rx)/2,
 					 DMA_DEV_TO_MEM,
 					 DMA_PREP_INTERRUPT);
+	if (!desc) {
+		dev_err(port->dev, "Preparing DMA cyclic failed\n");
+		goto chan_err;
+	}
 	desc->callback = atmel_complete_rx_dma;
 	desc->callback_param = port;
 	atmel_port->desc_rx = desc;