staging: comedi: gsc_hpdi: remove devpriv->hw_dev

The `hw_dev` member of `struct hpdi_private` is used to point to the
`struct pci_dev`.  This is redundant as the `struct comedi_device`
already has a pointer to the `struct device` within the `struct pci_dev`
and there is a convenient inline function, `comedi_to_pci_dev(dev)` that
returns a pointer to the `struct pci_dev`.

Remove the redundant `hw_dev` member and use alternate ways to get at
the `struct pci_dev`.

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