ath6kl: after cleanup properly reflect that sg is disabled

This allows the hardware to be restarted, as it will cause the
sg to be reinitialized.

Signed-off-by: James Minor <james.minor@ni.com>
Reviewed-by: Steve deRosier <steve.derosier@lairdtech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>