net/mqtt: cleanup TX function unref handling

With CONFIG_NET_DEBUG_NET_PKT enabled, the log messages are full of
errors regarding NULL parameters passed into net_pkt_unref in almost
every TX function for handling MQTT messages:
mqtt_tx_connect()
mqtt_tx_disconnect()
mqtt_tx_pub_msgs()
mqtt_tx_publish()
mqtt_tx_pingreq()
mqtt_tx_subscribe()
mqtt_tx_unsubscribe()
mqtt_parser()

Let's clean up the unref handling so that our debug log isn't quite
so full of error spam (as opposed to the normal amount of spam from
the net buf log).

Change-Id: Ib49d1192f51abe4329e6dc9da3c51a4a36290082
Signed-off-by: Michael Scott <michael.scott@linaro.org>