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>