fft_eval: updated drawn frequency range

According to Dave Aragon and his prior work, actually the 56 middle
carriers of the 64 subcarriers of the WiFi chip are used. My sample
data appears to be drawn more accurately using this calculation, so
use it by default.

Reported-by: Dave Aragon <daragon@uw.edu>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>