serial: sh-sci: Drop superfluous assignment in sci_request_dma()

No later code uses the assigned value, so it can be dropped.
Reported-by: NSimon Horman <horms@verge.net.au>
Fixes: 2c4ee235 ("serial: sh-sci: Postpone DMA release when falling back to PIO")
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1574,7 +1574,6 @@ static void sci_request_dma(struct uart_port *port)
 		if (dma_mapping_error(chan->device->dev, s->tx_dma_addr)) {
 			dev_warn(port->dev, "Failed mapping Tx DMA descriptor\n");
 			dma_release_channel(chan);
-			chan = NULL;
 		} else {
 			dev_dbg(port->dev, "%s: mapped %lu@%p to %pad\n",
 				__func__, UART_XMIT_SIZE,