staging: comedi: pcl818: introduce pcl818_ai_get_fifo_sample()

To clarify the code, introduce a helper function to read the analog
input data sample from the FIFO and optionally return the channel
that the sample was for. The channel is used to check for dropped
samples.

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>