mqtt: fix: Remove ping function call

Zephyr's MQTT does not implement a client's message queue.
So, a race-condition may be present when multiple messages are
received and processed.

We remove the pinreq function call to allow the mqtt_read function
to handle all the incoming messages.

CONFIG_IP_BUF_RX_SIZE is also updated to allow more messages to be
queued by the IP stack.

Jira: ZEP-669

Change-Id: Ie3ef55b17020e04c6540adf975a66fb004933914
Signed-off-by: Flavio Santes <flavio.santes@intel.com>