net: samples: Fix packet socket sample iface index

Patch (3206568e438dd97182bde66b8dafe4fbbffb330c) changed network
interface numbering starts from 1. The index 0 is reserved.
So use api to get default interface index for packet socket sample.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
1 file changed