hwmon: (wm831x-hwmon) Convert to use devm_ functions

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

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>