staging: comedi: das1800: tidy up analog output subdevice init

For aesthetics, add some whitespace to the subdevice init and rename
the (*insn_write) function.

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>