net/mqtt: fix race condition in mqtt_init()

We need to set the mqtt context recv function prior to calling
net_context_recv which installs the mqtt_recv callback.
If not, we risk a race condition where an unprotected reference
to mqtt->rcv(mqtt, buf) is made in mqtt_recv().

Change-Id: If90ee58f4ea6f7879ef7c12b969ba27647426acc
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>