thermal: db8500: Use of_match_ptr() macro in db8500_thermal.c

This eliminates having an #ifdef returning NULL for the case
when OF is disabled.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Hongbo Zhang <hongbo.zhang@stericsson.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>