enable selection of device based on the bus id using @<bus>

This was a key feature of the openni_camera drivers which allowed you to have multiple cameras, each on their own bus, and always get the one you wanted.

Given the different device strings, it's not implemented exactly the same as the openni_camera stack, which used "bus@index", here I assume you only have one device per bus (or at least, the first device on the bus will be used), and to keep the string parsing simple, the value is "@bus".

I've tested this with Asus Xtion Pro Live, 1.082 and 1.09 sensors in several configurations.