Do any poskim equate Christmas trees with the Ashera tree mentioned in the Gemara?
From a strictly halachic basis, no it is not an ashera. An ashera is a tree planted for the purpose of worshipping it. Today's commercial markets mean that big business grow these trees because they look nice and many people will buy them; and most people who buy them today have a pretty secularized version of Christmas anyhow.
Look: we know that many flavors of Christianity brought in some practices from the pagan world, so this may have been a related source at some point; and because the origins are pagan (and not entirely forgotten), it would be prohibited for Jews to put up a Christmas tree, because of "do not follow in their ways." But the average Christmas tree today is not a full-fledged halachic idol. (Nor is your average American family that decorates one "because that's what everyone does", who may or may not even go to church on Christmas, "worshipping idols.")
Suppose someone built a greenhouse specifically to grow "Christmas trees", and then he's moving out of town and wants to sell his greenhouse, which you'd like to buy to grow tomatoes. Would you argue that greenhouse was built specifically to house idols, and therefore you're now prohibited from using the greenhouse? I don't think so. What he had was a business selling nice-looking evergreen trees!