net: lib: coap_client: Don't decrease retry counter on send() failure

If send() fails, we have not technically send the CoAP retry yet, so
restore the same pending structure, so our timeouts and retry counters
stay the same.

This will trigger a retry next time the poll() return POLLOUT, so we
know that we can send.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>