staging: comedi: ii_pci20kc: remove forward declarations 2

Move the pci20xxx_dio_*() functions to remove the need for some of
the forward declarations.

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>