firmware: meson_sm: stop using 0 as NULL pointer

Use NULL for NULL pointer to fix the following sparse warning:
drivers/firmware/meson/meson_sm.c:85:24: sparse: warning: Using plain integer as NULL pointer

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230110031242.4917-1-miles.chen@mediatek.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>