I cannot get your question.
If you are connecting telem2 to the serial port on rpi-gpio, then you shuold check ttyAMA0, and there is no need to use a usb2serial converter.
And if you are connecting telem2 to usb port of rpi, then you need usb2serial converters such as ftdi, cp2102. they are mounted as ttyUSB*.
I am using pl2303hx to connect pixhawk to rpi, and it works fine in baudrate 921600.