staging: comedi: me4000: rename local variables used for 'dev->private'

In comedi drivers the local variable used for the dev->private pointer is
normally named 'devpriv'. For aesthetics, rename the variables in this
driver. Also, rename the struct to follow the norm.

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