drivers: udc_rpi_pico: fix set/clear endpoint halt

For the IN endpoint, we only need to set/reset the STALL bit in the
endpoint control register.
To set halt on the OUT endpoint, the AVAILABLE bit must also be set,
which is similar to starting a new transfer, but first any transfer in
progress must be canceled.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>