staging: comedi: adl_pci9111: cleanup pci9111_ao_insn_write()

Remove the unnecessary comment.

Only the last data value written needs to be cached for
readback.

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>