drivers: usb_dc_rpi_pico: Implemented vbus detection handling

As per USB 2.0 specification 7.1.5.1: The voltage source on the pull-up
resistor must be derived from or controlled by the power supplied on the
USB cable such that when VBUS is removed, the pull-up resistor does not
supply current on the data line to which it is attached.

Signed-off-by: Lukas Gehreke <lk.gehreke@gmail.com>