of: Fix property name in of_node_get_device_type

Commit 0413beda ("of: Add device_type access helper functions")
added a new helper not yet used in preparation for some treewide clean
up of accesses to 'device_type' properties. Unfortunately, there's an
error and 'type' was used for the property name. Fix this.

Fixes: 0413beda ("of: Add device_type access helper functions")
Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>