hwmon: (tmp421) introduce a channel struct

This is a preparatory change. Upcoming patches will introduce more
per-channel parameters so it's worth organizing them into a struct.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/baf34d95983a6b58a3e39e4c098e5979e541572e.1634029538.git.krzysztof.adamski@nokia.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>