You only need to look at Ryan8bit's avatar to see that Ricardo Montalban is pretty white and certainly not Indian. There is nothing wrong with Cumberbatch being Khan. For all we know Khan could have been raised in England to mixed parents or even had the genes responsible for looking like an indian removed as part of his genetic engineering. Maybe he was adopted into a family that had a surname of Khan? Just use your imagination. We know very very little about Khans background except he was a superhuman who once ruled a small empire on Earth.