staging: comedi: vmk80xx: consistently use the same local var names

Rename some of the local variables used in this driver to make the
code easier to maintain and understand.

s/udev/usb	the usb_device that the comedi_driver is attached to
s/dev/devpriv	the private data of the comedi_device
s/cdev/dev	the comedi_device

Also, use some local variables in a couple of the functions to tidy
up the code a bit.
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>