KVM: PPC: Book3s: Use arg->size directly in kvm_vm_ioctl_create_spapr_tce()

The size variable is just a copy of args->size, neither size nor args
are modifed, so just use args->size directly.

Signed-off-by: Deming Wang <wangdeming@inspur.com>
[mpe: Reword change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220703172932.11329-1-wangdeming@inspur.com