mkimage: do not write incorrect error message

When running 'mkimage -l' is called for a valid StarFive file an error
message "Error: invalid marker bytes" is written by the Renesas SPKG
driver.

mkimage -l may be invoked without specifying an image type. In this case
mkimage iterates over all image type drivers to find the one that matches.
None of the non-matching drivers should write an error message.

Fix the Renesas SPKG driver.

Fixes: afdfcb11 ("tools: spkgimage: add Renesas SPKG format")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>