staging: comedi: addi_apci_3120: tidy up CTR0 register defines

Define the CTR0 register in the main driver source file and remove all
the old defines in hwrdv_apci3120.c.

For aesthetics, save the raw digital output state (devpriv->do_bits) in
the digital output (*insn_bits) function and use a macro to set them when
reading/writing a timer.

Use the CTR0 register define in the digital output (*insn_bits) function
and remove the current register define.

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>