Bluetooth: controller: Allow to simply release node allocated in LLL

If we allocate a node in LLL and it turns out that we don't need it
(e.g. allocated connection on AUX_CONNECT_REQ, but connection handshake
did not complete and we don't need it) we still need to send it to ULL
to be released. We can use existing NODE_RX_TYPE_DC_PDU_RELEASE node
type for this purpose, but we just need to make sure it's passed from
LLL to ULL properly.

Also, since this now does not only release DC PDUs, let's change its
name to a more generic one.

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