usb: dwc3: gadget: Don't disconnect if not started

Don't go through soft-disconnection sequence if the controller hasn't
started. Otherwise, there will be timeout and warning reports from the
soft-disconnection flow.

Cc: stable@vger.kernel.org
Fixes: 61a34885

 ("usb: dwc3: gadget: Fix NULL pointer dereference in dwc3_gadget_suspend")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/linux-usb/20240215233536.7yejlj3zzkl23vjd@synopsys.com/T/#mb0661cd5f9272602af390c18392b9a36da4f96e6
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/e3be9b929934e0680a6f4b8f6eb11b18ae9c7e07.1708043922.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>