In function kwboot_xm_sendblock() the loop that implements retrying to
send a boot block might be quit if kwboot_tty_send or kwboot_tty_recv
return a failure. In this case the value of the variable c that is
expected to hold the response byte is uninitialized and most likely
still holds an ACK from the previous call to kwboot_xm_sendblock().

So the right thing to do is not to clobber rc if it's already != 0.

The result of this patch in my current scenario is that kwboot dies with

	xmodem: Connection timed out

when the SoC stops replying instead of continuing to try sending the
remaining blocks which results in select() blocking for one second for
each block.

Fixes: 0535713bbfa0 ("scripts: add kwboot tool")
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
--
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>

I also tried to increase the timeout, but without success :-(. Will test
a different USB-to-RS232 adapter ...
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>