pylibfdt: Rework "avoid unused variable warning" lines

Clang has -Wself-assign enabled by default under -Wall and so when
building with -Werror we would get an error here.  Inspired by Linux
kernel git commit a21151b9d81a ("tools/build: tweak unused value
workaround") make use of the fact that both Clang and GCC support
casting to `void` as the method to note that something is intentionally
unused.

Signed-off-by: Tom Rini <trini@konsulko.com>