Musicpal: Fix descriptor walk in eth_send

Commit 930c8682 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 2e87c5b9 tried to paper over.
Signed-off-by: NJan Kiszka <jan.kiszka@web.de>
Signed-off-by: Nmalc <av1474@comtv.ru>

hw/musicpal.c

@@ -238,14 +238,13 @@ static void eth_send(mv88w8618_eth_state *s, int queue_index)
 {
     uint32_t desc_addr = s->tx_queue[queue_index];
     mv88w8618_tx_desc desc;
+    uint32_t next_desc;
     uint8_t buf[2048];
     int len;
 
-    if (!desc_addr) {
-        return;
-    }
     do {
         eth_tx_desc_get(desc_addr, &desc);
+        next_desc = desc.next;
         if (desc.cmdstat & MP_ETH_TX_OWN) {
             len = desc.bytes;
             if (len < 2048) {
@@ -256,7 +255,7 @@ static void eth_send(mv88w8618_eth_state *s, int queue_index)
             s->icr |= 1 << (MP_ETH_IRQ_TXLO_BIT - queue_index);
             eth_tx_desc_put(desc_addr, &desc);
         }
-        desc_addr = desc.next;
+        desc_addr = next_desc;
     } while (desc_addr != s->tx_queue[queue_index]);
 }