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>