get_max_packet_size() implementation for dna

git-svn-id: https://svn.ntop.org/svn/ntop/trunk/PF_RING@8680 a02cd0c1-8e76-42e1-a119-56f9641475e2
Fix for initialization of ZC during driver attach/detach


git-svn-id: https://svn.ntop.org/svn/ntop/trunk/PF_RING@8684 a02cd0c1-8e76-42e1-a119-56f9641475e2