staging: comedi: dt2814: Remove struct dt2814_private

The `ntrig` member of the `struct dt2814_private` pointed to by
`dev->private` is no longer used as a counter to determine the end of
acquisition for a Comedi asynchronous command.  The other member
`curadscan` is also unused.  Remove the allocation of the private data
during initialization and remove the definition of `struct
dt2814_private` since they are no longer needed.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210301165757.243065-6-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>