hwmon: (max6642) Convert to use devm_ functions

Convert to use devm_ functions to reduce code size and simplify the code.

Cc: Per Dalen <per.dalen@appeartv.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>