usb: gadget: udc-core: fix wrong call order

commit 6d258a4c (usb: gadget: udc-core: stop UDC on device-initiated
disconnect) introduced another case of asymmetric calls when issuing
a device-initiated disconnect. Fix it.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>