staging: comedi: me4000: absorb ai_prepare()

This function never fails and it's only called by me4000_ai_do_cmd().

Absorb it and remove the unnecessary failure check.

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>