usb: ci13xxx_udc: Set gadget state to NOT_ATTACHED on disconnect

Currently we do not mark the gadget state to NOT_ATTACHED on
peripheral cable disconnect. Then if a floating charger cable is
connected, then the gadget state remains CONFIGURED and floating
charger detect work is not able to detect it as floating charger.

Fix this by marking gadget state to NOT_ATTACHED on cable
disconnect.

Change-Id: I53de2667dacf8cebc2e71a0b96b08f2ec48ebf48
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>