ath6kl: Rename ATH6KL_HW_FLAG_64BIT_RATES

Rename ATH6KL_HW_FLAG_64BIT_RATES to ATH6KL_HW_64BIT_RATES.
This seemed to be necessary to add/use new hardware flags
without exceeding 80 lines. We shall be adding new hw flags
dropping the FLAG term.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>