UPSTREAM: USB: cdc-wdm: sanitize error returns

UPSTREAM: USB: cdc-wdm: sanitize error returns

wdm_flush() returns unsanitized USB error codes.
They must be cleaned up to before being anded to user space

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 24a85bae5da2b43fed423859c09c5a81ab359473)

Change-Id: Ie56272c988fb62601bb50ce60753733467cc25f8
Signed-off-by: Ben Chan <benchan@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/35150
Reviewed-by: Olof Johansson <olofj@chromium.org>