staging: comedi: me_daq: fix me_ao_insn_write()

This function is supposed to write to a single analog output channel.
The channel number is packed in insn->chanspec, which is an unsigned
int, and unpacked using the CR_CHAN() macro.

Currently this function is trying to use the chanspec as an array.
This works only if a single value is written.

Fix the function so that the desired channel is determined and all
the data is written to that channel.

Also, fix the return. The comedi core expects insn_read functions to
return the number of data values (insn->n).

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>