The airlines aren't going to care what your diploma was in. All they're going to care about is your flying experience. By the time you reach airline age, you'll probably be in your early 30s... do you really think they're going to care what you read in a book when you were 19?
Diplomas/degrees are only of value for perhaps a networking advantage and when the computer in the HR department starts filtering who will be called to interviews (that is, assuming their "computer" is even using post secondary education as a filter).