dawn: fix network packet format to be host-agnostic
I tried Dawn and it whined at me because it didn't understand its
own packets from a different-endian host.
Mon Jun 8 10:49:12 2020 daemon.err dawn[19742]: not complete msg, len:308, expected len:872480768
Mon Jun 8 10:49:12 2020 daemon.err dawn[19742]: not complete msg, len:708, expected len:3288465408
https://github.com/berlin-open-wireless-lab/DAWN/issues/92
Signed-off-by: David Woodhouse <dwmw2@infradead.org>