qfw: Don't fail if setup data size is 0

Skip missing setup data (which is valid) rather than failing with an
error.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Reported-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>