The opposite direction is described in usb.tethering
USB reverse tethering is used to connect your Smartphone over your OpenWrt-Router with the Internet. Follow:
opkg install kmod-usb-net
usbN(where N is a number starting from 0) should appear. You can then configure it like any other ethernet interface, e.g.
ip a a 192.168.6.254 dev usb0or
ifconfig usb0 inet 192.168.6.254 up
Please note: Some smartphone, like e.g. the "HTC Desire" product family do not support Linux and atm it is not possible to use the above procedure, which is very simple and strait forward to connect your HTC Desire via USB via Linux to the Internet. For other OSes there is a special software called HTC sync. Me thinks the simplest solution is to not buy these products.