test: hexdump: fix misplaced return

One of the hexdump tests in test/lib/hexdump.c returns right at the
start of the function without testing anything.

Fix this by moving the 'return 0;' statement to the end of the function.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>