staging: kpc2000: added a helper to get struct kp2000_device from struct device.

The attribute call-backs all use the same formula to get the pcard from
dev:

  struct pci_dev *pdev = to_pci_dev(dev);
  struct kp2000_device *pcard;

  if (!pdev)
    return -ENXIO;
  pcard = pci_get_drvdata(pdev);
  if (!pcard)
    return -ENXIO;

Added a function to reduce the duplicated code.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>