staging: comedi: pcmda12: cleanup pcmda12_ao_insn_write()

Only the last value needs to be saved for readback.

Remove the LSB and MSB macros.

(*insn_write) 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>