Since cbw.Length can be less that sizeof(cbw.CDB), add code to zero
the whole struct out to avoid sending random stack data as a part of
payload. There's no known case where this causes a problem, but it's a
reasonable thing to do anyway.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>