Bluetooth: controller: Optimize calls to hci_get_class()

Cache the result of calling hci_get_class() to avoid repeatedly invoking
it on the same data. In order to cache it we take advantage of the fact
that both radio_pdu_node_rx_hdr and node_rx_hdr are not packed
structures and they currently have a spare padding byte (between type
and handle).

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>