Set mux value back to default when pinsets are released from the Portmux

[G33KatWork]

To avoid weird things happening, we should restore the default value in the portmux register for a pinset once the pins are released from the mux again.

It is debatable if this is necessary though: The GPIOs will still work and when a device that uses special function modes of pins like the UART is reinstatiated, a new Pinset that falls out of the Portmux-code needs to be passed into it which can only be created by calling mux() on it. This means the bits in the PORTMUX registers should always be set correctly no matter what happens.