KVM: PPC: e500mc: Fix a NULL dereference

We should set "err = -ENOMEM;", otherwise it means we're returning
ERR_PTR(0) which is NULL.  It results in a NULL pointer dereference in
the caller.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>