hwmon: (lm90) Define maximum number of channels that are supported

Use this define in all the places where literal '3' was used in this
context.

Signed-off-by: Slawomir Stepien <slawomir.stepien@nokia.com>
Link: https://lore.kernel.org/r/20220525073657.573327-6-sst@poczta.fm


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