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

For aesthetics, add some whitespace to the subdevice initialization
and rename the (*insn_write) functions to follow the normal form
in comedi drivers.

It's not possible to actually read the analog outpus. For convienence
the subdevice readback provided by the core is used to return the
last value written to the subdevice. Remove the SDF_READABLE flag
from the subdev_flags.

Async command support uses interrupts. For clarity, make sure the
driver has an IRQ before initializing command support.

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>