staging: comedi: ni_6527: use comedi attach_pci callback

Convert this PCI driver to use the comedi `attach_pci` callback instead
of the `attach` callback for PCI auto-configuration.  There is no need
to support manual attachment of PCI devices supported by this driver, so
remove the `attach` callback altogether.

Note that this driver still uses the list of PCI "mite" devices created
by the "mite" module.  This will be dealt with by a later patch once
dynamic allocation of "mite" structures has been implemented.

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