tools: kwboot: Fix handling of repeated xmodem packets

Unfortunately during some stages of xmodem transfer, A385 BootROM is not
able to handle repeated xmodem packets. So if an error occurs during that
stage, stop the transfer and return failure.

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