usb: dwc3: gadget: don't call stop_active_transfers() on disconnect

In case we get disconnected, we will call gadget
driver's disconnect method, which should make
sure to disable all endpoints. At that point
we will call stop_active_transfers() to make
sure we didn't leave any pending request on the
controller.

Tested-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>