USB: usbdevfs: restore warning for nonsensical flags

If we filter flags before they reach the core we need to generate our
own warnings.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Fixes: 0cb54a3e ("USB: debugging code shouldn't alter control flow")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>