drivers: udc_dwc2: Allocate at least 8 bytes for control OUT

Make sure to feed control OUT endpoint with at least 8 bytes buffer to
make it possible to always receive SETUP data. This solves the assertion
failure in net_buf_add() called inside dwc2_handle_evt_setup() when a
host decides to start new control transfer immediately after it has
issued control transfer with Data Stage from host to device with wLength
less than 8.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>