DynamicVertexBuffer.SetData() Fix

I stumbled across this doing a port of MonoGame to another platform.  I would expect that the SetData() method without the `SetDataOptions` parameter on Xbox 360 would always default to discard.

I don't think it would hurt things and the Windows DirectX version of MonoGame already defaults to discard if the VB is dynamic regardless of the `SetDataOptions` setting.

Any thoughts on this?