USB: cdc-wdm: sanitize error returns

commit 24a85bae5da2b43fed423859c09c5a81ab359473 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>