staging: comedi: comedidev.h: introduce comedi_to_pci_dev() helper

Introduce a wrapper for to_pci_dev() to allow the comedi_pci_drivers
to store the pci_dev pointer in the comedi_device hw_dev variable and
retrieve it easily.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbot <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>