Try to handle radiotap Rate values > 127 better.

Try to handle radiotap Rate values > 127 better.

Use the same heuristic Wireshark uses, i.e. assume that FreeBSD only
reports MCS indices between 0 and 15, and that any other values with the
0x80 bit set are rate values.  (Yes, those do exist.)

Also note, as I did in Wireshark, that it might be possible to extract
from the XChannel and Flags field the additional information to convert
an MCS index from the Rate field into a rate.  (Whether that's possible
depends on what platforms using the FreeBSD convention do with the
channel width and guard interval information.)