gpio: change Input<Disconnected> to Disconnected.

[Dirbaio]

This changes the default Pin state from Pin<Input<Disconnected>> to Pin<Disconnected>.

This is desirable because it makes is_high, is_low not available in Disconnected state, since they don't actually work. Nordic docs state this:

The input buffer of a GPIO pin can be disconnected from the pin to enable power savings when the pin is not used as an input, see GPIO port and the GPIO pin details. Inputs must be connected to get a valid input value in the IN register, and for the sense mechanism to get access to the pin.

I will add an into_disconnected function in a future PR, since it's desirable to disconnect input pins when not in use to save power.

[jonas-schievink]

Thanks! I thought you'd have to change more than that, neat.