FT857 and FT818 gpredict unstable

[mdblack98]

See csete/gpredict#326
FT818 and FT857 both have to swap VFOs and have to use EPROM reading to determine status.
Some timing issues are noted in the code.
So we'll try some ideas to see if we can get this more stable.
#1 First will be to try and determine that set_vfo actually worked by waiting to read the good vfo with a timeout.
#2 Next would be to only set VFOA during Rx and VFOB during Tx.