Bluetooth: Fix invalid response for 'Start Discovery' command

According to Management Interface API 'Start Discovery' command should
generate a Command Complete event on failure. Currently kernel is
sending Command Status on early errors. This results in userspace
ignoring such event due to invalid size.

bluetoothd[28499]: src/adapter.c:trigger_start_discovery()
bluetoothd[28499]: src/adapter.c:cancel_passive_scanning()
bluetoothd[28499]: src/adapter.c:start_discovery_timeout()
bluetoothd[28499]: src/adapter.c:start_discovery_complete() status 0x0a
bluetoothd[28499]: Wrong size of start discovery return parameters

Reported-by: Jukka Taimisto <jtt@codenomicon.com>
Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>