samples: net: socket: packet: Add SOCK_DGRAM support

If user has enabled SOCK_DGRAM support for AF_PACKET type, then
use that in the packet sample application instead of SOCK_RAW.
This simplifies the application as we do not need to handle
the Ethernet frame when sending or receiving the packets.

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