'Trådlös' is the usual way to say it. Though arguably '-lös' (-'less') could be construed as having a negative connotation, whereas '-fri' ('-free') has a positive one. That's probably why they went with 'Trådfri'.

Oh, "trådlös" is probably the most common one actually, now that you mention it. Completely forgot about that word for some reason.

It's hard to trademark common or idiomatic words. So using words that are almost right makes sense.