hwmon: (adt7470) Convert to use regmap

Convert the adt7470 to using regmap which allows better error handling.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20210826024121.15665-3-chris.packham@alliedtelesis.co.nzSigned-off-by: Guenter Roeck <linux@roeck-us.net>