staging: comedi: me4000: sample types are unsigned (and not long!)

Sample values in comedi are generally represented as unsigned values.
Change the types of various variables in the "me4000" driver dealing
with samples to use unsigned types for consistency.

Also replace the `long` or `unsigned long` variables used to handle
32-bit register values to `unsigned int`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>