staging: comedi: cb_pcimdda: cleanup the analog out read/write

Use the register map define to work out the i/o address.

Cleanup the comments about the simultaneous transfer mode for
the analog outputs.

Change the return to 'insn->n', the comedi core expects the return
to be the number of data elements used. Technically the 'i' value
is correct but 'insn->n' just makes it clearer.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>