ath6kl: Unset IFF_LOWER_UP flag on AP mode leave.

When disabling tethering after having enabled it, the ath6kl driver does
not set the IFF_LOWER_UP flag to 0 (carrier off) on the wlan interface.

The upper layers (eg. connman) are not notified of the tethering mode
status change. So, tethering can not be activated anymore.

This patch adds a netif_carrier_off() call when stopping AP mode to fix
the problem.

Signed-off-by: Pierre Le Magourou <plemagourou@aldebaran.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>