staging: comedi: store the 'index' for each subdevice

Store the 'index' for each comedi_subdevice when they are initially
allocated by comedi_alloc_subdevice(). This allows removing the
pointer math in comedi_fops.c which is used to figure out the
index that user space uses to access the individual subdevices.

Fix the ni_mio_common driver so it also uses the 'index' instead
of doing the pointer math.

Also, remove a couple unused macros in the pcmda12, pcmmio, and
pcmuio drivers which also do the pointer math to figure out the
index.

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