thermal: step_wise: Add missing static storage class specifiers

Fixes the following sparse warnings:
drivers/thermal/step_wise.c:153:5: warning:
symbol 'step_wise_throttle' was not declared. Should it be static?
drivers/thermal/step_wise.c:172:25: warning:
symbol 'thermal_gov_step_wise' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>