staging: comedi: poc: introduce struct poc_private

Wrap the private data used by this driver in a struct. This makes
the use of that data clearer and gets rid of the need for the casts.

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>