tools: kwboot: Change retry loop from decreasing to increasing

This patch does not change behavior of the code, just allows to implement
new changes more easily.

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