staging: comedi: usbduxfast: tidy up usbduxfastsub_ai_Irq()

Rename the CamelCase function,

Rename some of the local variables to the normal names used in
comedi drivers. Add a local variable for the comedi_async *.

Remove the sanity checking. The urb that causes this function to
be called can only be submitted if the sanity checks already
passed.

Change the way the comedi_subdevice is fetched. The attach function
sets the dev->read_subev pointer to the correct subdevice. Use that
instead of accessing the dev->subdevices array directly.

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>