comedi: ni_labpc_common: Conditionally remove I/O port support

In a future patch, the port I/O functions (`inb()`, `outb()`, and
friends will only be declared in the `HAS_IOPORT` configuration option
is enabled.

The ni_labpc_common module is used by the ni_labpc module (for ISA
cards), the ni_labpc_cs module (for PCMCIA cards), and the ni_labpc_pci
module (for PCI cards).  The ISA and PCMCIA cards use port I/O and the
PCI cards use memory-mapped I/O.

Conditionally compile the parts of the module that use the port I/O
functions so they are compiled if and only if the `CONFIG_HAS_IOPORT`
macro is defined, so that the module can be built if the port I/O
functions have not been declared.

Add a run-time check in the `labpc_common_attach()` to return an error
if the comedi device wants to use port I/O when the `CONFIG_HAS_IOPORT`
macro is undefined.

The changes allow the module to be built even if the port I/O functions
have not been declared.

Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20230913170712.111719-9-abbotti@mev.co.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/comedi/drivers/ni_labpc_common.c

@@ -78,6 +78,9 @@ static const struct comedi_lrange range_labpc_ao = {
  * functions that do inb/outb and readb/writeb so we can use
  * function pointers to decide which to use
  */
+
+#ifdef CONFIG_HAS_IOPORT
+
 static unsigned int labpc_inb(struct comedi_device *dev, unsigned long reg)
 {
 	return inb(dev->iobase + reg);
@@ -89,6 +92,8 @@ static void labpc_outb(struct comedi_device *dev,
 	outb(byte, dev->iobase + reg);
 }
 
+#endif	/* CONFIG_HAS_IOPORT */
+
 static unsigned int labpc_readb(struct comedi_device *dev, unsigned long reg)
 {
 	return readb(dev->mmio + reg);
@@ -1200,8 +1205,12 @@ int labpc_common_attach(struct comedi_device *dev,
 		devpriv->read_byte = labpc_readb;
 		devpriv->write_byte = labpc_writeb;
 	} else {
+#ifdef CONFIG_HAS_IOPORT
 		devpriv->read_byte = labpc_inb;
 		devpriv->write_byte = labpc_outb;
+#else
+		return -ENXIO;
+#endif
 	}
 
 	/* initialize board's command registers */