staging: comedi: pcl818: factor out the common "next ai chan" code

The DMA, FIFO, and EOC interrupt handlers all have common code that
bumps the counters in the private data that keep track of what channel
is being sampled next and when all the data has been sampled.

Factor this common code into a helper function.

Make sure the COMEDI_CB_BLOCK event is set when advancing to the next
channel so that when the comedi_event() is finally called the core will
wake the async_queue to return the analog input data.

Also, make sure the COMEDI_CB_EOS event is set at the completion of each
scan.

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>