thermal: Move get_thermal_instance to the internal header

thermal: Move get_thermal_instance to the internal header

The function is not used any place other than the thermal
directory. It does not make sense to export its definition in the
global header as there is no use of it.

Move the definition to the internal header and allow better
self-encapsulation.

Take the opportunity to add the parameter names to make checkpatch
happy and remove the pointless stubs.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-6-daniel.lezcano@linaro.org