Revert "Revert "usb: gadget: udc: core: Invoke usb_gadget_connect only when started""

This reverts commit ea56ede9.

It breaks the Android KABI and will be brought back at a later time when
it is safe to do so.

Bug: 161946584
Change-Id: I9c46b16a91915da08bf5300cb3d273d243e6a893
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>