tools: kwboot: Fix kwboot_xm_sendblock() function when kwboot_tty_recv() fails

tools: kwboot: Fix kwboot_xm_sendblock() function when kwboot_tty_recv() fails

When kwboot_tty_recv() fails or times out, it does not set the `c`
variable to NAK. The variable is then compared, while it holds either
an undefined value or a value from previous iteration. Set `c` to NAK so
that the other side will try to resend current block, and remove the
now unnecessary break.

In other failure cases return immediately.

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