staging: comedi: rti800: factor out "ai wait for conversion"

Factor the timeout loop that waits for the ai conversion to complete
out of rti800_ai_insn_read().

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>