staging: comedi: das08: Use struct comedi_device hw_dev for PCI

Remove the pointer to the PCI device from the private data `struct
das08_private_struct`.  Use `comedi_set_hw_dev()` to save a pointer to
the PCI device (actually, its embedded `struct device`) and
`comedi_to_pci_dev()` to retrieve it.

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