staging: comedi: vmk80xx: use auto_attach() hook

Use the new `auto_attach()` hook in the `struct comedi_driver` instead
of the old `attach_usb()` hook.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>