log: websocket: Fix log output function return value

The log output function registered for the logger backend is expected to
return the number of bytes processed, but currently it returns the TX
retry count. This commit fixes it. In case data could only partially be
sent, respective bytes count is returned. In case of TX error, the bytes
are assumed dropped to prevent busy looping in the logger.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>