usb: gadget: dummy_hcd: fixup error probe path

If USB2 host controller probes fine but USB3 does not then we don't
remove the USB controller properly and lock up the system while the HUB
code will try to enumerate the USB2 controller and access memory which
is no longer available in case the dummy_hcd was compiled as a module.

This is a problem since 448b6eb1 ("USB: Make sure to fetch the BOS desc
for roothubs.) if used in USB3 mode because dummy does not provide this
descriptor and explodes later.

Cc: stable@vger.kernel.org # v3.5
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>