hwmon: constify pointers to hwmon_channel_info

HWmon core receives an array of pointers to hwmon_channel_info and it
does not modify it, thus it can be array of const pointers for safety.
This allows drivers to make them also const.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>