don't hand roll bpf_mtap_hdr functionality, just use bpf_mtap_hdr.
the radiotap code prepends a big struct to the packets, and wires
them up with the packet by putting an mbuf on the stack and using
that as the head of an mbuf chain. bpf_mtap_hdr does the chain head
thing for us, so shrink this code by calling the bpf function.
there's some other drivers that do this too, so if anyone wants a
free commit they should go looking in the other wireless drivers
and do the same change.
ok claudio@
UnifiedSplitRaw