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: 82295bc0 ("spi: s3c64xx: move dma_release_channel to unprepare")
Fixes: f52b03c7 ("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.comSigned-off-by: Mark Brown <broonie@kernel.org>