spi: s3c64xx: correct dma_chan pointer initialization

Use NULL for dma channel pointer initialization instead of plain integer.

sparse warnings: (new ones prefixed by >>)
>> drivers/spi/spi-s3c64xx.c:387:34: sparse: sparse: Using plain integer as NULL pointer
   drivers/spi/spi-s3c64xx.c:388:34: sparse: sparse: Using plain integer as NULL pointer

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 82295bc0d1 ("spi: s3c64xx: move dma_release_channel to unprepare")
Fixes: f52b03c707 ("spi: s3c64xx: requests spi-dma channel only during data transfer")
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Andi Shyti <andi@etezian.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220808004851.25122-1-chanho61.park@samsung.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-s3c64xx.c

@@ -389,8 +389,8 @@ static int s3c64xx_spi_unprepare_transfer(struct spi_master *spi)
 	if (sdd->rx_dma.ch && sdd->tx_dma.ch) {
 		dma_release_channel(sdd->rx_dma.ch);
 		dma_release_channel(sdd->tx_dma.ch);
-		sdd->rx_dma.ch = 0;
-		sdd->tx_dma.ch = 0;
+		sdd->rx_dma.ch = NULL;
+		sdd->tx_dma.ch = NULL;
 	}
 
 	return 0;