asm-generic/io.h: warn in inb() and friends with undefined PCI_IOBASE

asm-generic/io.h: warn in inb() and friends with undefined PCI_IOBASE

When PCI_IOBASE is not defined, it is set to 0 such that it is ignored
in calls to the readX/writeX primitives. This triggers clang's
-Wnull-pointer-arithmetic warning and will result in illegal accesses on
platforms that do not support I/O ports.

Make things explicit and silence the warning by letting inb() and
friends fail with WARN_ONCE() and a 0xff... return in case PCI_IOBASE is
not defined.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Link: https://lore.kernel.org/lkml/20210421111759.2059976-1-schnelle@linux.ibm.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>