hwmon: (adt7475) Add attenuator bypass support

Added support for reading DTS properties to set attenuators on
device probe for the ADT7473, ADT7475, ADT7476, and ADT7490.

Signed-off-by: Logan Shaw <logan.shaw@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
[groeck: Continuation line formatting; dev_err -> dev_warn]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>