drivers: bluetooth: hci_nxp: fix nomem handling

When allocation fails in `hci_rx_cb`, we should return immediately
instead of continuing the execution of the function.

Also, we need to free the allocated buffer from the heap when sending
to the message queue fails to avoid memory leak.

Signed-off-by: Axel Le Bourhis <axel.lebourhis@nxp.com>