spi: add function to release SPI device

include/spi.h

@@ -14,6 +14,7 @@ struct spi_dev {
 };
 
 struct spi_dev *spi_probe(void);
+void spi_release(struct spi_dev *dev);
 int spi_set_cs_level(struct spi_dev *dev, int level);
 int spi_tx_rx(struct spi_dev *dev, void *rx_buf, const void *tx_buf,
 	size_t len);

source/drivers/spi_flash.c

@@ -152,3 +152,9 @@ err_free_dev:
 	free(dev);
 	return NULL;
 }
+
+void flash_release(struct flash_dev *dev)
+{
+	spi_release(dev->priv);
+	free(dev);
+}

source/drivers/stm32f0_spi.c

@@ -62,6 +62,11 @@ struct spi_dev *spi_probe(void)
 	return dev;
 }
 
+void spi_release(struct spi_dev *dev)
+{
+	free(dev);
+}
+
 static int stm32f0_spi_set_cs_level(struct spi_dev *dev, int level)
 {
 	(void)dev;