hwmon: (amc6821) Use BIT() and GENMASK()

Use BIT() and GENMASK() for bit and mask definitions
to help distinguish bit and mask definitions from other
defines and to make the code easier to read.

No functional change intended.

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