staging: comedi: addi_apci_2200: cleanup digital output subdevice

The board supported by this driver always has a digital output subdevice.
Remove the boardinfo for it and just open-code the relevant data in the
subdevice init.

Remove the SDF_GROUND and SDF_COMMON from the subdevice 'subdev_flags'.
These flags only have meaning for analog input/output subdevices. Also,
remove the SDF_READABLE flag, it is not required by output only subdevices.

Remove the subdevice 'len_chanlist' initialization. This variable only
has meaning for subdevices that support asynchronous commands.

Remove the subdevice 'io_bits' initialization. Digital output subdevices
don't use this variable.

Remove the subdevice function pointers that evaluate to NULL based on the
boardinfo data.

Move the apci2200_do_insn_bits() function from the hwdrv_apci2200.c file
into the main driver file.

For aesthetic reasons, rename the #define used for the register used to
read/write the digital outputs..

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>