ath9k: Add cast to u8 to FREQ2FBIN macro

The macro results are assigned to u8 variables/fields. Adding the cast
fixes plenty of clang warnings about "implicit conversion from 'int' to
'u8'".

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>