staging: comedi: me4000: remove ai context

The ai context is a struct containing all the register addresses
used with the ai subdevice. These can easily be calculated when
needed. Remove the me4000_ai_context struct, its intialization
function, and it's variable in the private data.

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>