hwmon: (amc6821) Convert to use regmap

Use regmap for register accesses and caching.

While at it, use sysfs_emit() instead of sprintf() to write sysfs
attribute data, and remove spurious debug messages which would only
be seen as result of a bug in the code. Also make sure that error
codes are propagated and not replaced with -EIO.

While at it, introduce rounding of written temperature values and for
internal calculations to reduce deviation from written values and as
much as possible.

No functional change intended except for differences introduced by
rounding.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>