usb: add USB_MSG_PLUG_OUT event

When the core received an USB_MSG_PLUG_OUT event, it will stop all the
classes. This make a chance that the classes could get rid off doing
useless stuff while the USB cable is plugged out.
usbdevice/core: cope with some buggy drivers on USB_MSG_DATA_NOTIFY

Some driver will emit USB_MSG_DATA_NOTIFY before the USB get configured.
Filter out those events.
Merge pull request #100 from grissiom/usb-plug

usb: add USB_MSG_PLUG_OUT event