serial: sh-sci: do not warn if DMA transfers are not supported

Not all (H)SCIF devices support DMA, and failure to set it up is not
normally a cause for concern. This patch demotes the associated warning to
debug output.

Inspired by BSP patch "sci: sh-sci: Fix transfer sequence of unsupport DMA
transfer" (6beb1f98d3bd30) by Hiromitsu Yamasaki.
Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1516,7 +1516,7 @@ static struct dma_chan *sci_request_dma_chan(struct uart_port *port,
 	chan = dma_request_slave_channel(port->dev,
 					 dir == DMA_MEM_TO_DEV ? "tx" : "rx");
 	if (!chan) {
-		dev_warn(port->dev, "dma_request_slave_channel failed\n");
+		dev_dbg(port->dev, "dma_request_slave_channel failed\n");
 		return NULL;
 	}