libfdt: Correctly handle versions > 17

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> If future dtb version > 17 are defined, that are still backwards
> compatible with v16, libfdt will of course be able to read and
> understand them.  However, when modifying such a tree, it can't
> guarantee that it won't clobber additional structure from the new
> version which it doesn't know about.  Therefore, before making
> modifications to a tree of version >17, we must change it's version to
> be exactly 17.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl