net: mqtt: Return -1 if keepalive messages are disabled.

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>