kernel: fix kernel builds for chipidea w/ usb-gadget

Instead of repackaging udc-core.ko again, just add a dependency to
kmod-usb-gadget, which already contains it.

Fixes kmod-chipidea builds for 3.10- with usb-gadget enabled.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 41115