staging: comedi: dt282x: fix dt282x_ao_insn_read()

The comedi core expects the (*insn_read) functions to return 'insn->n'
samples. Fix this function to work like the core expects.

For aesthetics, rename the private data 'ao' member to clarify its
use. Also, remove the unnecessary comment.

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>