bluetooth: controller: refactor node_rx_pdu for memory optimization

Generic node_rx_hdr struct contains data not used for done event, so this
pdu specific data is moved to node_rx_pdu to minimize memory usage for
MFIFO_done alloc.

This however calls for a considerable write-through replacing the generic
node_rx_hdr with node_rx_pdu when relevant and needed.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>