spi: img-spfi: fix potential double release

The channels spfi->tx_ch and spfi->rx_ch are not set to NULL after they
are released. As a result, they will be released again, either on the
error handling branch in the same function or in the corresponding
remove function, i.e. img_spfi_remove(). This patch fixes the bug by
setting the two members to NULL.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/1573007769-20131-1-git-send-email-bianpan2016@163.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-img-spfi.c

@@ -673,6 +673,8 @@ static int img_spfi_probe(struct platform_device *pdev)
 			dma_release_channel(spfi->tx_ch);
 		if (spfi->rx_ch)
 			dma_release_channel(spfi->rx_ch);
+		spfi->tx_ch = NULL;
+		spfi->rx_ch = NULL;
 		dev_warn(spfi->dev, "Failed to get DMA channels, falling back to PIO mode\n");
 	} else {
 		master->dma_tx = spfi->tx_ch;