Bluetooth: Fix not checking for valid hdev on bt_dev_{info,warn,err,dbg}

This fixes attemting to print hdev->name directly which causes them to
print an error:

kernel: read_version:367: (efault): sock 000000006a3008f2

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>