tools: kwboot: Fix return type of kwboot_xm_makeblock() function

tools: kwboot: Fix return type of kwboot_xm_makeblock() function

Function kwboot_xm_makeblock() always returns length of xmodem block. It
is always non-negative and calculated from variable with size_t type. Set
return type of this function to size_t and remove dead code which checks
for negative value.

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