hwmon: (w83627ehf) Disable setting DC mode for pwm2, pwm3 on NCT6776F

NCT6776F only supports pwm mode for pwm2 and pwm3. Return error if an attempt
is made to set those pwm channels to DC mode.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@vger.kernel.org # 3.0+
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>