Bluetooth: Fix notifying mgmt power off before flushing connection list

This patch moves the mgmt_powered() notification earlier in the
hci_dev_do_close() function. This way the correct "not powered" error
gets passed to any pending mgmt commands. Without the patch the pending
commands would instead get a misleading "disconnected" response when
powering down the adapter.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>