usb/ehci: Guard definition of EHCI_DEBUG

Guard against re-definition of EHCI_DEBUG. Allows for turning on of debug info
from configure (using --qemu-extra-cflags="-DEHCI_DEBUG=1") rather than source
code hacking.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-ehci.c

@@ -36,7 +36,9 @@
 #include "dma.h"
 #include "sysemu.h"
 
+#ifndef EHCI_DEBUG
 #define EHCI_DEBUG   0
+#endif
 
 #if EHCI_DEBUG
 #define DPRINTF printf