staging: comedi: adv_pci_dio: post increment 'subdev' in (*auto_attach)

For aesthetics, post-increment the 'subdev' index when used to get a
comedi_subdevice pointer instead of incrementing it after the subdevice
is initialized.

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