net: cadence_gem: Fix top comment

To indicate Cadence GEM not Xilinx.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/net/cadence_gem.c

 /*
- * QEMU Xilinx GEM emulation
+ * QEMU Cadence GEM emulation
  *
  * Copyright (c) 2011 Xilinx, Inc.
  *