net: mqtt: Close the socket right after sending disconnect message

After sending the MQTT disconnect message, no response is expected,
therefore it makes little sense to delay the socket closure.

So far it was expected to call `mqtt_input` function after calling
`mqtt_disconnect` in order to close the socket, which is
counter-intuitive. Simplify this, by closing the socket rightaway
in the `mqtt_disconnect` function.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>