powerpc/secvar: Fix incorrect return in secvar_sysfs_load()

secvar_ops->get_next() returns -ENOENT when there are no more variables
to return, which is expected behaviour.

Fix this by returning 0 if get_next() returns -ENOENT.

This fixes an issue introduced in commit bd5d9c74 ("powerpc: expose
secure variables to userspace via sysfs"), but the return code of
secvar_sysfs_load() was never checked so this issue never mattered.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-4-ajd@linux.ibm.com