xilinx_axienet: avoid useless self-assignment

Statement s=s; makes little sense, remove it. Spotted by Clang
compiler.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/xilinx_axienet.c

diff --git a/hw/xilinx_axienet.c b/hw/xilinx_axienet.c
index adfaf2c..9b08c62 100644
--- a/hw/xilinx_axienet.c
+++ b/hw/xilinx_axienet.c
@@ -648,7 +648,6 @@
     uint16_t csum16;
     int i;
 
-    s = s;
     DENET(qemu_log("%s: %zd bytes\n", __func__, size));
 
     unicast = ~buf[0] & 0x1;