comedi: comedi_8254: Conditionally remove I/O port support

The comedi_8254 module supports both port I/O and memory-mapped I/O.
In a future patch, the port I/O functions (`inb()`, `outb()`, and
friends) will only be declared if the `HAS_IOPORT` configuration option
is enabled.

Conditionally compile the parts of the module that use port I/O so they
are compiled if and only if the `CONFIG_HAS_IOPORT` macro is defined, so
that it can still be built if the port I/O functions have not been
declared.  If `CONFIG_HAS_IOPORT` is undefined, replace the GPL-exported
`comedi_8254_io_alloc()` function with a dummy static inline version
that just returns `ERR_PTR(-ENXIO)`.

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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