USB: fix problem with usbtmc driver not loading properly

The usbtmc driver forgot to export its device table to userspace.
Without this, it is never loaded properly when such a device is seen by
the system.

Cc: Marcel Janssen <marcel.janssen@admesy.nl>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>