net: samples: Use lighter printk() instead of printf()

Use printk(), snprintk() instead of printf() and snprintf().
CONFIG_STDOUT_CONSOLE is anyway disabled by default so printf()
will not output anything without it.

Change-Id: Ide24e4d669e9821289189ed28e30a596120d9611
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>