spi: properly exchange bytes by sending and reading each byte within a SPI transaction

source/drivers/stm32f0_spi.c

@@ -75,11 +75,13 @@ static int stm32f0_spi_tx_rx(struct spi_dev *dev, void *rx_buf, size_t rx_len,
 	gpio_clear(GPIOA, GPIO4);
 
 	for (i = 0; i < tx_len; ++i) {
-		spi_send8(dev->dev_id, tx ? *tx++ : 0);
+		spi_send8(dev->dev_id, *tx++);
+		(void)spi_read8(dev->dev_id);
 	}
 
 	if (rx_buf) {
 		for (i = 0; i < rx_len; ++i) {
+			spi_send8(dev->dev_id, 0);
 			*rx++ = spi_read8(dev->dev_id);
 		}
 	}