ath9k: reduce stack usage in ar9003_aic_cal_post_process

In some configurations, this function uses more than the warning limit
of 1024 bytes:

drivers/net/wireless/ath/ath9k/ar9003_aic.c: In function 'ar9003_aic_cal_post_process':
drivers/net/wireless/ath/ath9k/ar9003_aic.c:434:1: error: the frame size of 1040 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

It turns out that there are two large arrays on the stack here, but
almost all the data in them is never used outside of the loop in
which it gets written, so we can replace the array with a single
instance.

The .valid flag is used later, so I'm replacing the array of structures
with an array of bools. An obvious follow-up optimization would be
to replace it with a bitmask and set_bit()/find_first_bit()/
find_last_bit()/... operations. However, I have not tested this patch,
so I sticked to the simpler transformation that does the job of
reducing the stack usage to a harmless level.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>