Previously, the function would always return the max_len parameter.
This poses two issues:
1. the API requires to return the actual packet size
2. the API requires that if the packet is larger than max_len, the
packet is flushed and -ENOBUFS is returned
3. this basically bypasses the packet flushing, consequtive _recv()
would return the last packet again
This commit fixes those issues.
Kaspar Schleiser
70d480c301