Musicpal: Fix descriptor walk in eth_send

Commit 930c86820e introduced a regression to eth_send: eth_tx_desc_put
manipulates the host's tx descriptor copy before writing it back, but
two lines down the descriptor is evaluated again, leaving us with an
invalid next address if host and guest endianness differ. So this was
the actual issue commit 2e87c5b937 tried to paper over.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: malc <av1474@comtv.ru>
