hwmon: (adt7475) Add support for inverting pwm output

Add a "adi,pwm-active-state" device-tree property to allow hardware
designs to use inverted logic on the PWM output.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
[groeck: dev_err -> dev_warn]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>