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

drivers/comedi/drivers/comedi_8254.c

···
 #include <linux/comedi/comedidev.h>
 #include <linux/comedi/comedi_8254.h>
 
+#ifdef CONFIG_HAS_IOPORT
+
 static unsigned int i8254_io8_cb(struct comedi_8254 *i8254, int dir,
 				unsigned int reg, unsigned int val)
 {
···
 		return inl(iobase + reg_offset);
 	}
 }
+
+#endif	/* CONFIG_HAS_IOPORT */
 
 static unsigned int i8254_mmio8_cb(struct comedi_8254 *i8254, int dir,
 				   unsigned int reg, unsigned int val)
···
 	return i8254;
 }
 
+#ifdef CONFIG_HAS_IOPORT
+
 /**
  * comedi_8254_io_alloc - allocate and initialize the 8254 device for pio access
  * @iobase:	port I/O base address
···
 	return __i8254_init(iocb, iobase, osc_base, iosize, regshift);
 }
 EXPORT_SYMBOL_GPL(comedi_8254_io_alloc);
+
+#endif	/* CONFIG_HAS_IOPORT */
 
 /**
  * comedi_8254_mm_alloc - allocate and initialize the 8254 device for mmio access

include/linux/comedi/comedi_8254.h

···
 #define _COMEDI_8254_H
 
 #include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/err.h>
 
 struct comedi_device;
 struct comedi_insn;
···
 void comedi_8254_subdevice_init(struct comedi_subdevice *s,
 				struct comedi_8254 *i8254);
 
+#ifdef CONFIG_HAS_IOPORT
 struct comedi_8254 *comedi_8254_io_alloc(unsigned long iobase,
 					 unsigned int osc_base,
 					 unsigned int iosize,
 					 unsigned int regshift);
+#else
+static inline struct comedi_8254 *comedi_8254_io_alloc(unsigned long iobase,
+						       unsigned int osc_base,
+						       unsigned int iosize,
+						       unsigned int regshift)
+{
+	return ERR_PTR(-ENXIO);
+}
+#endif
+
 struct comedi_8254 *comedi_8254_mm_alloc(void __iomem *mmio,
 					 unsigned int osc_base,
 					 unsigned int iosize,