samples: net: Fix VLAN tag name in prj.conf file

The prj.conf file was using wrong VLAN tag option names.
The correct names are CONFIG_SAMPLE_VLAN_TAG and
CONFIG_SAMPLE_VLAN_TAG_2 as defined in Kconfig file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>