serial: imx: also update RX stats in DMA path

The RX bytecount was only updated in the PIO path and thus
the device erroneously reported a value of 0 if DMA is in
use.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -918,6 +918,7 @@ static void dma_rx_callback(void *data)
 				sport->port.icount.buf_overrun++;
 		}
 		tty_flip_buffer_push(port);
+		sport->port.icount.rx += count;
 	}
 
 	/*