Bluetooth: Use NULL instead of integer for mgmt_device_connected param

Last param of mgmt_device_connected is of pointer type, so use NULL
instead of 0 for it. This fix following sparse warning:

CHECK   net/bluetooth/hci_event.c
net/bluetooth/hci_event.c:3262:74: warning: Using plain integer as NULL pointer

Signed-off-by: Szymon Janc <szymon@janc.net.pl>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>