hwmon: (tmp421) introduce MAX_CHANNELS define

There are few places where the maximal number of channels is used define
the size of arrays but when raw number is used it is not clear that they
really related to this quantity.
This commit introduces MAX_CHANNELS define and uses it those places to
give some context to the number.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/abc1a213a25b890b799b35ad94bb543a2ade7fc8.1632473318.git.krzysztof.adamski@nokia.com


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