spapr_llan: Fix warning when compiled with -dDEBUG

Compiling with the DEBUG macro causes leaves hw/spapr_llan.c with an
unused variable, which is treated as an error in the qemu build.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>