zabbix: fix 6in4 network interface discovery

Those devices don't have 'device' propery, only 'l3_device', which
causes 'attempt to concatenate field 'device' (a nil value)' lua error.

Use 'l3_device' as a fallback in this case.

Signed-off-by: Jacek Konieczny <jajcus@jajcus.net>