Bluetooth: host: Use a linked list instead of a FIFO as the RX queue

There is no need to use a k_fifo object to queue the items that are
passed to bt_recv() now that we are using a work queue instead of a
thread, since there is no need for blocking on the actual queue, instead
relying on the fact that work is triggered to know that an item is ready
for processing.

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