serial: stm32: correct loop for dma error handling

In this error handling, "transmit_chars_dma" function will call
"transmit_chars_pio" once per characters. But "transmit_chars_pio" will
continue to send characters while xmit buffer is not empty.

Remove this useless loop, one call is sufficient.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220104182445.4195-5-valentin.caron@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: VCASTM

drivers/tty/serial/stm32-usart.c

@@ -477,7 +477,7 @@ static void stm32_usart_transmit_chars_dma(struct uart_port *port)
 	const struct stm32_usart_offsets *ofs = &stm32port->info->ofs;
 	struct circ_buf *xmit = &port->state->xmit;
 	struct dma_async_tx_descriptor *desc = NULL;
-	unsigned int count, i;
+	unsigned int count;
 
 	if (stm32_usart_tx_dma_started(stm32port)) {
 		if (!stm32_usart_tx_dma_enabled(stm32port))
@@ -542,8 +542,7 @@ static void stm32_usart_transmit_chars_dma(struct uart_port *port)
 	return;
 
 fallback_err:
-	for (i = count; i > 0; i--)
-		stm32_usart_transmit_chars_pio(port);
+	stm32_usart_transmit_chars_pio(port);
 }
 
 static void stm32_usart_transmit_chars(struct uart_port *port)