[PATCH] librtlsdr: conditional USB transfer resubmit

This patch implements zero-copy overhead for buffers used for sample transfer. This improves a bit performance for applications where low overhead is required and can save a bit of memory. Two changes are made: * add return value for callback function * add function for buffer release when it is free for reuse This change breaks ABI (callback returns int instead of void), but I cannot find way not to do so. Proper change of library version in CMaleList.txt would be required. Please comment on it and let my now if it is interesting enough to be merged upstream. JP