staging: comedi: adv_pci1710: remove 'ai_data_len' from private data

This member of the private data is just 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>