thermal/core: Move parameter validation from __thermal_zone_get_temp to thermal_zone_get_temp

All callers of __thermal_zone_get_temp() already validated the
thermal zone parameters. Move validation to thermal_zone_get_temp()
where it is actually needed. Also add kernel documentation for
__thermal_zone_get_temp(), listing the requirement that the
function must be called with validated parameters and with thermal
device mutex held.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>