comedi: comedi_8254: Use a call-back function for register access

Rework the comedi_8254 module to use a call-back function for register
access.  This will make it easier to isolate the parts that will depend
on the `CONFIG_HAS_IOPORT` macro being defined and also allows the
possibility of supplying an external callback function during
initialization by a variant of the `comedi_8254_init()` and
`comedi_8254_mm_init()` functions, although that has not been
implemented yet.

The `struct comedi_8254` members have been changed to use a pointer to a
callback function and a context of type `unsigned long`.  The
`comedi_8254_init()` and `comedi_8254_mm_init()` functions use an
internal callback function and set the context to the base address of
the registers (for `comedi_8254_mm_init()` that involves converting a
`void __iomem *` to `unsigned long`).

A minor change to `dio200_subdev_8254_offset()` in the
amplc_dio200_common module has been made due to the changes in `struct
comedi_8254`.

Cc: Arnd Bergmann <[email protected]>
Cc: Niklas Schnelle <[email protected]>
Signed-off-by: Ian Abbott <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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