hwmon: (pmbus/max31827) Explain why enum chips must not start with 0

hwmon: (pmbus/max31827) Explain why enum chips must not start with 0

If a driver calls device_get_match_data(), the .data pointer in its id
data structures must not be NULL/0 because device_get_match_data()
returns NULL if an entry is not found. Explain that in a comment to avoid
confusion why this is required in this driver but not in other drivers.

Cc: Daniel Matyas <daniel.matyas@analog.com>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>