fix register_channel setting of expiry

[pjenvey]

register_channel attempts to SET the expiry field but it's UpdateExpression is an ADD, which is documented to: "If the attribute is a number, and the value you are adding is also a number, then the value is mathematically added to the existing attribute".

this should be ADD chids :channel_id SET expiry :expiry instead. The rust side already has a fix included in: #1229