staging: comedi: amplc_dio200: absorb dio200_pci_common_attach()

Absorb `dio200_pci_common_attach()` into `dio200_attach_pci()` since
that's the only place it is called from.

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