riscv: io: avoid null-pointer arithmetic in PIO helpers

riscv: io: avoid null-pointer arithmetic in PIO helpers

When port I/O is not supported, exposing the port-string helpers is both
unnecessary and can make clang diagnose null-pointer arithmetic from the
PCI_IOBASE based address expression.  Keep the MMIO string helpers
available as before, but only provide the port I/O variants when
CONFIG_HAS_IOPORT is enabled.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260703122832.15984-2-cuiyunhui@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>