samples/net: Fix logging format error

When building the echo_client sample app, the following warning
is found:

echo-client.c:37:
echo_client/src/echo-client.c: In function 'send_ipv4_data':
include/logging/sys_log.h:108:20: warning: format '%d' expects argument
                                  of type 'int', but argument 7 has
                                  type 'size_t' ...

So, this patch changes %d by %zu to remove that warning.

Change-Id: I8a3822bca6d5988e531684d647de93bd77aabf9b
Signed-off-by: Flavio Santes <flavio.santes@intel.com>