Bluetooth: controller: Silence unused variable warning with non-AE

Places definition of lll under the ADV_EXT #ifdef in lll_adv.c:isr_tx,
to silence build warnings for non-AE builds.

Follow-up from #29753.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>