Just follow the regular setup for generic Linux on their website: Linux USB Server | VirtualHere. You’ll need the MIPSEL build, or if you want to go one step further, use the optimised MIPSEL-24Kc (but that is license bound).
OpenWRT also includes the more standard usbip software, which you could use for this purpose - it has clients not just for OpenWrt, but generic Linux (mainline kernel support since 3.17) and Windows as well. Plus, it’s completely free and open source. This is the guide for setting it up though parts of it might be outdated.