dwc_otg exports usb_gadget_probe_driver function and patches out the udc-core module which normally exports it. This breaks other USB device controller drivers, most notably the virtual dummy_hcd ones, and causes the kernel compile to throw a load of errors if you try to build dwc_otg and gadget at the same time.

OpenWRT patched this issue:

https://lists.openwrt.org/pipermail/openwrt-devel/2012-November/017427.html