UBUNTU: [Config] Enable CONFIG_USB_GADGET_DUMMY_HCD

The PC platform does not contain the correct hardware to be a USB slave,
so having CONFIG_USB_GADGET_NET2280 enabled does not make much sense. See
email discussion at:

https://lists.ubuntu.com/archives/kernel-team/2009-February/004447.html

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>