powerpc/eeh: Skip eeh sysfs when eeh is disabled

When eeh is not enabled, and hotplug two pci devices on the same bus, eeh
related sysfs would be added twice for the first added pci device. Since the
eeh_dev is not created when eeh is not enabled.

This patch adds the check, if eeh is not enabled, eeh sysfs will not be
created.

After applying this patch, following warnings are reduced:

sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:00.0/eeh_mode'
sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:00.0/eeh_config_addr'
sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:00.0/eeh_pe_config_addr'

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>