serial: sprd: Fix a copy-paste err in sprd_request_dma()

when dma_request_chan rx_dma fails, dev_err log should print rx_dma
err log.

Signed-off-by: YueHaibing <[e-mail]>
Reviewed-by: Mukesh Ojha <[e-mail]>
Reviewed-by: Baolin Wang <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/sprd_serial.c

···
 	sp->rx_dma.chn = dma_request_chan(port->dev, "rx");
 	if (IS_ERR(sp->rx_dma.chn)) {
 		dev_err(port->dev, "request RX DMA channel failed, ret = %ld\n",
-			PTR_ERR(sp->tx_dma.chn));
+			PTR_ERR(sp->rx_dma.chn));
 		sp->rx_dma.enable = false;
 	}
 }