ath9k: fix and simplify FFT max index retrieval

FFT max index retrieval was not retrieved correctly for HT20/HT40 FFT
frames. Fixing the retrieval allows us to remove the fixup function as
well. While at it, split the spectral_max_index function into versions
for ht20 and ht40 to simplify the code.

Cc: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>