usb: gadget: udc-core: call ->disconnect() when soft disconnecting

when disconnecting from host using our soft_connect
sysfs interface, also let the gadget driver know about
it by calling the gadget driver's ->disconnect()
method.

No problems have been found while not calling ->disconnect()
so far, it's just good convention.

Signed-off-by: Felipe Balbi <balbi@ti.com>