Now we correctly set the GiveBack flag and minimum current fields in the
Request message when GiveBack is enabled. Also when GiveBack is
enabled, we correctly handle GotoMin messages by lowering our power
consumption, completing negotiation, and Requesting our normal power
periodically until the power supply Accepts our Request.
At least, I believe it's correct. I don't have any hardware that sends
GotoMin messages, so I can't test it other than that it doesn't break
anything with the GiveBack flag set to 1 or 0 when the source doesn't
ask to reduce power.