net: samples: Unref net_buf using net_nbuf_unref

Using net_buf_unref() technically works but debugging the network buffer
allocations is more difficult if done like that.

Change-Id: Ib8e3f8b412c2f8388315c2f63cae4392f814ea2f
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>