hwmon: (adm9240) Convert to regmap

Convert the adm9240 driver to using regmap and add error handling.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20200924085102.15219-4-chris.packham@alliedtelesis.co.nz
[groeck: Fixed context conflict against 'hwmon: use simple i2c probe function']
Signed-off-by: Guenter Roeck <linux@roeck-us.net>