samples: net: cloud: Fix polling incoming messages

Topic subscribe() will allow cloud to send messages to
device. There will be a bit of network delay. But mqtt_input
was called only after publish() which will trigger every
10-15 seconds. Which is causing more delay to read published
messages from cloud even though messages are already available
at socket level.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>