ath9k: Skip malformed frames on normal FFT report

Since we have lots of frames on a normal FFT report don't bother
processing the malformed ones. Only try to fix malformed frames
in case of a short FFT report (only a single frame on the report).

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