usb: gadget: pch_udc: Fix usb/gadget/pch_udc: Fix ether gadget connect/disconnect issue

ISSUE:
After a USB cable is connect/disconnected, the system rarely freezes.

CAUSE:
Since the USB device controller cannot know to disconnect the USB cable, when
it is used without detecting VBUS by GPIO, the UDC driver does not notify to
USB Gadget.

Since USB Gadget cannot know to disconnect, a false setting occurred when the
USB cable is connected/disconnect repeatedly.

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>