500Hz pwm

Hi Steffen,

please consider adding support for 500Hz pwm rc signal in blheli main code. I've recently did a brushless conversion for V400D02 and I struggeled to get blheli working on main. The problem was that Walkera's RX2614V-D has 500Hz pwm on throttle output and blheli was detecting it as PPM. 

Thanks,
Eugene