tools: kwboot: Remove code for handling CAN byte

It is unknown why handling of CAN byte was added into kwboot tool as
Marvell BootROM does not support CAN byte. It never sends CAN byte to host
and if host sends CAN byte BootROM handles it as an unknown byte.

Remove code for handling and sending CAN bytes from the kwboot tool.

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