Hartmut went into detail on how to get an Atheros wireless adapter working in hostap mode: https://forum.opnsense.org/index.php?topic=20956.0
My question is why hasn't this been implemented in the Web GUI? Wireless interfaces also seem to be missing some configuration options, such as HT mode (only ht40 is enabled in the web GUI). This is fixable in the command line, but it would be nice to have more web GUI options for wireless cards.