staging: comedi: pcl816: tidy up the analog input (*insn_read)

For aesthetics, move this function out of the async command support
code.

For safety, the INT request (end-of-conversion flag) should be cleared
before doing each conversion and after the final data sample is read.
This driver does that but it's a bit awkward with the initial clear being
outside the for loop that reads the samples.

Refactor the function a bit so it's more like the pcl818 driver and we
can use common code to clear the flag for a timeout and after the last
sample.

Do a bit of other tidying up during the move.

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>