Are you an AT&T customer with a grandfathered unlimited data plan? If so, you may want to check your bill to make sure that’s still the plan you’re paying for, reports the Houston Chronicle. In 2009, AT&T adopted new, tiered data plans for smartphone users, dropping its traditional unlimited data plans for new customers. Those who already had the plans could keep them, AT&T said, even if they got a new device or signed a new contract…