iot/zoap: Ignore non-request packets in zoap_handle_request

For example, when a RESET packet is passed to zoap_handle_request(),
there's nothing it can do, and it's not an error, so it returns
success silently.

Change-Id: I025bb44733521d6132999c219aaa292a3de302d7
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>