Bluetooth: Controller: Relax radio packet pointer assignment deadline

Relax the radio packet pointer assignment deadline assertion
until access address being transmitted. The PDU buffer is
probably only needed just after access address is being
transmitted or received by the radio. This will give some
more breathing room for slow CPUs like in nRF51x SoCs.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>