ath6kl: convert ar6004 hardware flags to firmware feature flags

The functionality defined through these flags were actually firmware features
which can change between firmware versions. To make it possible to support
different firmware versions with the same driver, convert the flags to firmware
feature flags.

For backwards compatibility support for old ar6004 firmware FW
API 3 or smaller images we forcefully set the feature bits in the driver.
Starting from FW API 5 the firmware image needs to set them.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>