ath6kl: call ath6kl_cfg80211_stop() from ath6kl_close()

This way it's possible to keep all disconnect logic in one function and
easier to add new functionality, like stopping scheduled scan.

There are some changes to commands called during network interface close,
but there should not be any visible changes in functionality.

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