staging: comedi: pcmmio: cleanup the digital i/o register defines

Redefine the registers used to access the digital i/o so that they
are based on the dev->iobase of the board instead of the 'asic_iobase'
that is stored in the private data.

Remove the then unused 'asic_iobase'.

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>