tools: kwbimage: Add missing check for maximal value for DATA_DELAY

Data delay is stored as 8-bit number in kwbimage structure. Ensure the
given value is at most 255.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>