thermal: sun8i: Fix using plain integer as NULL pointer in sun8i_ths_resource_init

sparse returns a warning:

"drivers/thermal/sun8i_thermal.c:341:60: sparse: sparse: Using plain
integer as NULL pointer".

Fix it by replacing the zero integer by a NULL pointer.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200112171318.23025-1-tiny.windzz@gmail.com