mqtt: FIX: add unsubscribe function and update return codes

MQTT unsubscribe message was not included previously. See ZEP-623.
So, this commit adds the handling functions for the MQTT UNSUBSCRIBE
message.

MQTT high-level API return codes are now unified to only return:
- 0 on success
- -EINVAL when invalid parameters are received
- -EIO on network error

API documentation is also updated to reflect the previous changes.

Jira: ZEP-623

Change-Id: I04d65c303762ce2ecaca73a4f222f0b77fe70503
Signed-off-by: Flavio Santes <flavio.santes@intel.com>