staging: comedi: pcl711: factor out the ai end-of-conversion wait

Factor out the analog input end-of-conversion wait to clarify the
pcl711_ai_insn() function a bit.

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>