In mqtt_keepalive_time_left(), return -1 if keep alive messages are
disabled by setting CONFIG_MQTT_KEEPALIVE=0.
This allows to use mqtt_keepalive_time_left() directly as an input
for poll(). If no keep-alive is expected, -1 would indicate
that poll() can block until new data is available on the socket.
Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>
