[bluez] Do not report a mode change is adapter is already down

It may happen that hciops receives HCI adapter down event first and a
command complete event for read scan enable command after that;
informing adapter of a mode change after the interface is already down
may result in adapter code getting stuck in off state.

Fixed by not calling adapter_mode_changed() if interface is down.
Merge pull request #45 from hmallat/do-not-mode-change-when-down

[bluez] Do not report a mode change is adapter is already down