rtt: moved isRunning() check from TaskContext::dataOnPort() to default TaskContext::dataOnPortHook()

This allow the user to decide in an overwritten dataOnPortHook() whether the user callbacks should be
enqueued, indepedent of whether the component is running or not. Note that the callbacks are not executed
until the component starts unless #70 is merged.

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>