ath9k: Support processing of multiple FFT frames per report.

Since we can identify the FFT frames on the report by checking the
consistency of their magnitude info, we can process all of them,
even when some of them are corrupted.

This patch introduces two functions to quickly verify the integrity
of the mag_info fields and some further tweaks to detect the frames
in the report and process them.

Note that in case of missing bytes we don't duplicate them, instead
we leave them as zeroes. This way we get less noise on the FFT plot.

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