staging: comedi: amplc_dio200: use custom register access functions

Add custom functions to access hardware registers and call them instead
of calling `inb()` and `outb()` directly.  When additional cards are
supported by this driver requiring different register access methods,
the new functions will localize the register access differences to just
these functions.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>