net: samples: Fix Rx buffers count.

Incase of slip or ethenet use, if these is too much incoming data
current buffer count is not sufficient. Noticed with TCP client
and server example. So increasing it to avoid not enough Rx buffers
issue.

Change-Id: I6eef51c28c7c4751955aa4403122bf17d55ce12c
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>