ath9k: return when short FFT frame was handled

With the loop break like this, there are false "FFT report truncated"
messages because the iterator is not advanced as the check expects.

Instead, just return, for a single frame there is nothing left to be
done anyways.

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