ath9k: Fix hanlding of maximum magnitude index

Maximum magnitude index is a 5bit signed integer,
convert to an 8bit signed integer and then "shift" it so that it can be used
as an array index. Note that the current implementation adds +1 to the index
value (so it can't be used as an array index) and it's only valid for HT20
channels.

Note that the maximum magnitude index is not being used by
the userspace tools that parse FFT samples (they just use
maximum magnitude) so this doesn't break userspace compatibility.

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