ath9k: Perform integrity checks when processing FFT frames

a) Check that the maximum magnitude is at the specified index

b) Check if the maximum magnitude index is at dc_pos and if so
calculate a new one (value at dc_pos is invalid)

c) Check if the specified maximum magnitude is indeed the maximum

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>