powerpc/eeh: Fix build error caused by pci_dn

eeh.h could be included when we have following condition. Then we
run into build error as below: (CONFIG_PPC64 && !CONFIG_EEH) ||
(!CONFIG_PPC64 && !CONFIG_EEH)

In file included from arch/powerpc/kernel/of_platform.c:30:0:
./arch/powerpc/include/asm/eeh.h:344:48: error: ‘struct pci_dn’ \
declared inside parameter list [-Werror]
    :
In file included from arch/powerpc/mm/hash_utils_64.c:49:0:
./arch/powerpc/include/asm/eeh.h:344:48: error: ‘struct pci_dn’ \
declared inside parameter list [-Werror]

This fixes the issue by replacing those empty inline functions
with macro so that we don't rely on @pci_dn when CONFIG_EEH is
disabled.

Cc: stable@vger.kernel.org # v4.1+
Fixes: ff57b454 ("powerpc/eeh: Do probe on pci_dn")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>