samples/net: Remove call to unref routine when net_send returns >= 0

According to last comments posted in ZEP-632, the unref routine must
be called only when net_send reports an error.

Jira: ZEP-632
Change-Id: I11553edf2c7d686c3e58617c2e05b6f18962955e
Signed-off-by: Flavio Santes <flavio.santes@intel.com>
4 files changed