ath9k: Use ARRAY_SIZE macro

Use ARRAY_SIZE macro, rather than explicitly coding some variant of it
yourself.
Found with: find -type f -name "*.c" -o -name "*.h" | xargs perl -p -i -e
's/\bsizeof\s*\(\s*(\w+)\s*\)\s*\ /\s*sizeof\s*\(\s*\1\s*\[\s*0\s*\]\s*\)
/ARRAY_SIZE(\1)/g' and manual check/verification.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>