staging: comedi: pcmda12: cleanup pcmda12_ao_insn_read()

Remove the boilerplate comment from the 'skel' driver.

To initiate the simultaneaous transfer, only one analog output
register needs to be read. Move the read out of the for() loop.

(*insn_read) functions should return an errno or the number of
samples actually read. Change the final return to insn->n to
make this clear.

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>