util-ioctl: only get MTU when iface name is set

This patch fixes a warning message when suricata is started without
giving an interface name on the command line. The code was trying
to get the MTU even if pcap_dev was not set.

PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/125
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/64