Don't call the poll callback unnecessarily.

Don't call the poll callback unnecessarily.

Currently we call the poll callback if there is any free space in the send
buffer, every time we call utcp_timeout(). However, if the application
does not send anything inside the callback, then it most likely won't send
anything the next time if the free space in the send buffer did not change.
So only call the callback when more space has been made available in the
send buffer.