spi: s3c64xx: set pointers to null using NULL rather than 0

There are pointers being set to null using use. Use NULL instead.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Link: https://lore.kernel.org/r/20220612203428.2754823-1-colin.i.king@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>