ath9k: remove repetitions of mask array size

The constant "123", which is the number of elements in
mask_m / mask_p, is repeated several times in this function.

Replace memsets with array initialization, and replace a loop
conditional with ARRAY_SIZE() so that we don't repeat ourselves.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>