Fix publish with topic ID >=127

[embhorn]

Fixes zd16708

• While sending MQTT-SN Publish, if normal topic id is less than or equal to 127, publish is done correctly on the expected topic id.
• If topic id is equal to or greater than 128, some garbage value is sent in topic ID in publish packet instead of the given topic id. Example, when we try to publish on Normal topic ID 128, Publish is sent on topic ID 65408 instead.

Fixed by proper cast of word16 in SN_Encode_Publish

Also fixes topic_type set in SN_Decode_Publish