staging: comedi: 8255: use pointer to dev->subdevices

Instead of accessing the dev->subdevices directly as an array,
use a pointer. This method is more common in the comedi subsystem.

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