[Blackfin] serial driver: Fix bug Free rx dma buffer in shutdown.

Kernel crash for the serial driver in DMA mode:
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3679

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>

drivers/serial/bfin_5xx.c

@@ -704,6 +704,7 @@ static void bfin_serial_shutdown(struct uart_port *port)
 	disable_dma(uart->rx_dma_channel);
 	free_dma(uart->rx_dma_channel);
 	del_timer(&(uart->rx_dma_timer));
+	dma_free_coherent(NULL, PAGE_SIZE, uart->rx_dma_buf.buf, 0);
 #else
 #ifdef	CONFIG_KGDB_UART
 	if (uart->port.line != CONFIG_KGDB_UART_PORT)