net: net_send_data() should not free the net_buf

It is expected that the caller of the net_send_data() function
will release the net_buf or do whatever it wants with it if
there is an error while sending the net_buf.

Change-Id: Id9ddeed3da8b5091c0a8b9d72377f9d12fa76b59
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>