staging: comedi: pcl812: tidy up the digital subdevices

For aesthetics, tidy up the digtial subdevice code.

rename the register map defines for the digital input and output ports.

Move the two digital subdevice (*insn_bits) functions out of the analog
support functions. Tidy them up a bit during the move.

Add some whitespace to the subdevice init.

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>