usb: dwc3: Notify gadget driver upon vbus disconnect

Currently when usb_gadget_vbus_disconnect() is called,
dwc3_gadget_disconnect_interrupt() is only called if the device is not in
CONFIGURED state. However this does not capture the case when a disconnect
occurs quickly after a connect event, resulting in the gadget driver
missing the notification. Instead, simply notify about the disconnect
regardless of configured status.

CRs-fixed: 682476
Change-Id: I35a5d3f57a415ebeccd780281b41c5846fa5a9f6
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>