staging: comedi: gsc_hpdi: use auto_attach method

This driver does not need to support manual attachment of supported PCI
devices.  Replace the `attach()` hook (`hpdi_attach()`) with an
`auto_attach()` hook (`hpdi_auto_attach()`).  This will be called via
`comedi_pci_auto_config()` at PCI probe time.

This driver no longer increments the PCI reference during attachment, so
remove the call to `pci_dev_put()` when detaching the device.

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