Bluetooth: Controller: Simplify aux flush

aux->rx_last cannot be NULL since it's always set after acquire to a
valid node so the flow in flush() can be simplified.

rx parameter is only used to update PDU chain, then it's overwritten,
so we can instead update PDU chain in caller since there's only one
place when this should happen.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>