net: cadence_gem: Fix Tx descriptor update

The local variable "desc" was being used to read-modify-write the
first descriptor (of a multi-desc packet) upon packet completion.
desc however continues to be used by the code as the current
descriptor. Give this first desc RMW it's own local variable to
avoid trampling.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/net/cadence_gem.c

@@ -911,14 +911,16 @@ static void gem_transmit(GemState *s)
 
         /* Last descriptor for this packet; hand the whole thing off */
         if (tx_desc_get_last(desc)) {
+            unsigned    desc_first[2];
+
             /* Modify the 1st descriptor of this packet to be owned by
              * the processor.
              */
-            cpu_physical_memory_read(s->tx_desc_addr,
-                                     (uint8_t *)&desc[0], sizeof(desc));
-            tx_desc_set_used(desc);
-            cpu_physical_memory_write(s->tx_desc_addr,
-                                      (uint8_t *)&desc[0], sizeof(desc));
+            cpu_physical_memory_read(s->tx_desc_addr, (uint8_t *)desc_first,
+                                     sizeof(desc_first));
+            tx_desc_set_used(desc_first);
+            cpu_physical_memory_write(s->tx_desc_addr, (uint8_t *)desc_first,
+                                      sizeof(desc_first));
             /* Advance the hardare current descriptor past this packet */
             if (tx_desc_get_wrap(desc)) {
                 s->tx_desc_addr = s->regs[GEM_TXQBASE];