ath9k: fix reporting calculated new FFT upper max

Since the debug print code is outside of the loop, it shouldn't use the loop
iterator anymore but instead print the found maximum index.

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