tty/serial: at91: correct buffer size used in DMA

The buffer size set in DMA is inconsistent with its allocation.
So keep them consistent here. The structure atmel_uart_char is
used in PIO mode with its meaning. But here in DMA, all of the
buffer is treated as general char.
Signed-off-by: NLeilei Zhao <leilei.zhao@atmel.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/atmel_serial.c

@@ -1030,7 +1030,7 @@ static int atmel_prepare_rx_dma(struct uart_port *port)
 	BUG_ON(!PAGE_ALIGNED(ring->buf));
 	sg_set_page(&atmel_port->sg_rx,
 		    virt_to_page(ring->buf),
-		    ATMEL_SERIAL_RINGSIZE,
+		    sizeof(struct atmel_uart_char) * ATMEL_SERIAL_RINGSIZE,
 		    (int)ring->buf & ~PAGE_MASK);
 	nent = dma_map_sg(port->dev,
 			  &atmel_port->sg_rx,