samples: net: secure_mqtt: error code to enum

The error code `0x80` is replaced with its
corresponding `mqtt_suback_return_code` enum
value. This will make the code more readable, as
users of this example do not have to look up, what
`0x80` encodes.

Signed-off-by: Pályi Lőrinc <palyilorinc33@gmail.com>