samples: net: stats: Update project configuration

Fix two issues with the current configuration:
* The log buffer was too small for the sample, causing part of the stats
  printout to be dropped
* The sample set CONFIG_NET_CONFIG_NEED_IPV4 but did not actually set
  any IPv4 address, nor enable DHCP, causing the NET_CONFIG
  initialization failure.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>