kernel: fix kernel build for chipidea w/o usb-gadget

This resolves a module dependency kernel regression in r40905 with the
usb-chipidea module if usb-gadget was not selected.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40992 3c298f89-4303-0410-b956-a3cf2f4a3e73