staging: comedi: pcl818: factor out the common "dropout" detect code

The DMA, FIFO, and EOC interrupt handlers all have common code that
checks for channel dropout when running an async command.

Factor this common code into a helper function.

Only return the sample if the channel is valid. The EOC handler was
previously returning the value then checking for channel dropout.

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>