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

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@gibson.dropbear.id.au>