I have gone through the source code of Wireshark 2.4.1. In this i found that, decoding of the 'Tx Attenuation' and 'Tx Attenuation ion dB' fields are of ENC_BIG_ENDIAN. But, in the official link of radio tap header http://www.radiotap.org/, i didn't find anything specified about the byte ordering.
Note: Except these two and FHSS fields, rest of the fields are using ENC_LITTLE_ENDIAN encoding. Which one is correct? LITTLE or BIG? can you provide the reference for that information?