ath6kl: add firmware capabilities support

The new firmware format includes capability bits which make it
possible to check what features the firmware supports. Add infrastructure
to read the capabilities. For now it only provides
ATH6KL_FW_CAPABILITY_HOST_P2P which is not even used anywhere yet, but that
will be added later.

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