staging: comedi: rtd520: use auto_attach instead of attach_pci

Change the rtd520 driver to use the new `auto_attach()` method in
`struct comedi_driver` instead of the `attach_pci()` method.  I plan to
remove the `attach_pci()` and `attach_usb()` methods once nothing is
using them.

Tag the new `rtd520_auto_attach()` function definition with `__devinit`
as it is only called during PCI probe.

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