ehea: Register memory hotplug, reboot and crash hooks on adapter probe

ehea creates memory hotplug, reboot and crash hooks even if there
are no adapters in the box. Just create them when we probe our
first adapter.

[cascardo: use ehea_register_memory_hooks return code]
Signed-off-by: Anton Blanchard <anton@samba.org>
Tested-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>