net: mqtt: mqtt_live should only return success when ping sent

Users of mqtt_live() have no idea when it actually sends a ping.
As a result it's very hard to know when to use mqtt_input() to
process the incoming PINGACK.

Instead of returning a 0 result when a ping isn't generated in
mqtt_live(), let's return -EAGAIN.

Signed-off-by: Michael Scott <mike@foundries.io>