powerpc/powernv Adapt opal-elog and opal-dump to new sysfs_remove_file_self

We are currently using sysfs_schedule_callback() which is deprecated
and about to be removed. Switch to the new interface instead.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>