dtc: Remove gcc 4.6 "set but not used" warnings

[Jon Loeliger]

> A number of the dtc testcases trigger the new "variable set but not
> used" warning from gcc 4.6.  That is they have variables which are
> assigned, but then never read after that point.
> 
> In a couple of cases this is just because the variables aren't needed,
> so this patch removes them.  In subnode_offset.c, it's because one
> pair of variables we clearly intended to test we don't actually test.
> This patch also adds this missing check.
> 
> This patch makes the testsuite compile clean with gcc 4.6.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl