disk: convert DT_NODE_HAS_PROP(..,label) to DT_HAS_NODE()

The check for label property is really checking to see if a device
exists and the better way to do that is to use DT_HAS_NODE().  Replace
refernces of DT_NODE_HAS_PROP(DT_INST(...), label) with
DT_HAS_NODE(DT_INST(...)).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>