staging: comedi: adl_pci6208: use attach_pci callback

Convert this PCI driver to use the comedi PCI auto config attach mechanism
by adding an attach_pci callback function. Since the driver does not require
any external configuration options, disable the legacy attach by making the
attach callback simply return -ENOSYS. This removes the need to walk the pci
bus to find the pci_dev and the need for the pci_dev_put() in the detach.

For aesthetic reasons, rename the local variable 'thisboard' to 'boardinfo'.

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