rio_get_device — Begin or continue searching for a RIO device by vid/did
RIO vid to match or RIO_ANY_ID to match all vids
RIO did to match or RIO_ANY_ID to match all dids
Previous RIO device found in search, or NULL for new search
Iterates through the list of known RIO devices. If a RIO device is found with a matching vid and did, the reference count to the device is incrememted and a pointer to its device structure is returned. Otherwise, NULL is returned. A new search is initiated by passing NULL to the from argument. Otherwise, if from is not NULL, searches continue from next device on the global list. The reference count for from is always decremented if it is not NULL.