staging: comedi: addi_common.h: remove 'ui_AiDataLength' from private data

This member of the private data is a copy of the s->async->prealloc_bufsz.
Use that instead.

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>