hwmon: (nct6775) Fix writing pwmX_mode

pwmX_mode is defined in the ABI as 0=DC mode, 1=pwm mode. The chip
register bit is set to 1 for DC mode. This got mixed up, and writing
1 into pwmX_mode resulted in DC mode enabled. Fix it up by using
the ABI definition throughout the driver for consistency.

Fixes: 77eb5b37

 ("hwmon: (nct6775) Add support for pwm, pwm_mode, ... ")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>