usb: gadget: udc-core: stop UDC on device-initiated disconnect

When we want to do device-initiated disconnect,
let's make sure we stop the UDC in order to
e.g. allow lower power states to be achieved by
turning off unnecessary clocks and/or stoping
PHYs.

When reconnecting, call ->udc_start() again to
make sure UDC is reinitialized.

Cc: stable@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>