staging: comedi: pcl818: introduce pcl818_ai_write_sample()

This driver can acquire analog input samples during the async command with
DMA, by using the FIFO, or sample-by-sample using the End-Of-Conversion
interrupt. All three methods do the following sequence:

  1) check for channel dropout
  2) add the sample to the async buffer
  3) advance the channel dropout detection and detect the end of the command

Merge this sequence into a new helper function.

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>