hw/net: Fix the transmission return size

Fix the transmission return size because not all bytes could be
transmitted successfully. So, return a successful length instead of a
constant value.

Signed-off-by: Fea.Wang <fea.wang@sifive.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
diff --git a/hw/net/xilinx_axienet.c b/hw/net/xilinx_axienet.c
index 7d1fd37..05d41bd 100644
--- a/hw/net/xilinx_axienet.c
+++ b/hw/net/xilinx_axienet.c
@@ -847,7 +847,7 @@
     axienet_eth_rx_notify(s);
 
     enet_update_irq(s);
-    return size;
+    return s->rxpos;
 }
 
 static size_t