[media] siano: simplify message endianness logic

Currently, every time a message is sent or received, the endiannes
need to be fixed on big endian machines. This is currently done
on every call to the send API, and on every msg reception logic.
Instead of doing that, move it to the send/receive functions.
That simplifies the logic and avoids the risk of forgetting to
fix it somewhere.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>