staging: comedi: me4000: move ai command timing values into private data

The ai (*do_cmd_test) calls me4000_ai_round_cmd_args() to calculate the
timing values needed to the command. The the command test passes, the
core will then call the ai (*do_cmd) which then has to call
me4000_ai_round_cmd_args() again in order to get the same values to
pass to ai_prepare() in order to program the timing.

Add members to the private data to allow the (*do_cmd_test) to calculate
and save to values needed by ai_prepare().

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>