KVM: PPC: Book 3S: Fix error return in kvm_vm_ioctl_create_spapr_tce()

Fix to return error code -ENOMEM from the memory alloc error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>