tools: kwboot: Fix wrong parameter passed to read()

tools: kwboot: Fix wrong parameter passed to read()

The 'buf' variable is a pointer and '_buf' is the array itself.
Therefore we should pass sizeof(_buf) instead of sizeof(buf) to read().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>