tty: 8250_dma: use dmaengine_prep_slave_sg()

This is a preparatory for the serial-to-kfifo switch. kfifo understands
only scatter-gatter approach, so switch to that.

No functional change intended, it's just dmaengine_prep_slave_single()
inline expanded.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240405060826.2521-10-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dma.c

@@ -89,6 +89,7 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 	struct circ_buf			*xmit = &p->port.state->xmit;
 	struct dma_async_tx_descriptor	*desc;
 	struct uart_port		*up = &p->port;
+	struct scatterlist sg;
 	int ret;
 
 	if (dma->tx_running) {
@@ -111,10 +112,13 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 
 	serial8250_do_prepare_tx_dma(p);
 
-	desc = dmaengine_prep_slave_single(dma->txchan,
-					   dma->tx_addr + xmit->tail,
-					   dma->tx_size, DMA_MEM_TO_DEV,
-					   DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+	sg_init_table(&sg, 1);
+	sg_dma_address(&sg) = dma->tx_addr + xmit->tail;
+	sg_dma_len(&sg) = dma->tx_size;
+
+	desc = dmaengine_prep_slave_sg(dma->txchan, &sg, 1,
+				       DMA_MEM_TO_DEV,
+				       DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 	if (!desc) {
 		ret = -EBUSY;
 		goto err;