The ftdi chip itself has a "get bitmode" command that doesn't actually
return the current bitmode, just a snapshot of the gpio lines. The chip
apparently has no way to provide the current bitmode.
This implements the functionality at the driver level. The driver starts
out assuming the chip is in UART mode (which it will be, coming out of
reset) and keeps track of every successful set-bitmode operation so that
it can always return the current mode with UFTDIIOC_GET_BITMODE.
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