staging: comedi: cleanup comedi_alloc_subdevices

Access the individual comedi_subdevices using a pointer instead
of directly accessing as an array. This is how the rest of the
comedi core accesses them.

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