net/dp8393x: disable user creation

Netcard needs an address space to write data to, which can't be specified
on command line.
This fixes a crash when user starts QEMU with "-device dp8393x"
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Reviewed-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>

hw/net/dp8393x.c

@@ -889,6 +889,8 @@ static void dp8393x_class_init(ObjectClass *klass, void *data)
     dc->reset = dp8393x_reset;
     dc->vmsd = &vmstate_dp8393x;
     dc->props = dp8393x_properties;
+    /* Reason: dma_mr property can't be set */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo dp8393x_info = {