Add RIG_LEVEL_USB_AF

[mdblack98]

Many rigs (e.g. IC7300) have the ability to set the USB AF level controlling audio over the USB cable to the rig.
This usually includes the ACC jack.
Adding this will allow WSJTX and JTDX and others to create their own AGC control by adjusting the USB AF level first and then the RF gain as a backup to reduce audio.
Ideally for FT8 modes and such the RF gain should be maxed out, computer audio at 0dB, and adjust the USB AF level to get the proper audio level of 70dB in WJSTX and 82dB in JTDX (RMS and Peak meter respectively) as a minimum value.

Rigs added so far
IC-705
IC-7300
IC_7610
IC-9700
IC-785X
IC-R8600