drivers: bluetooth: hci: rpmsg: fix handling of hci events

Fixed handling of HCI events in the HCI driver over RPMsg. Now,
the driver makes use of discardable buffer pool when allocating
memory for certain HCI event types (e.g. Advertising Report Event).
Applications that are flooded with Advertising Reports will run
much better after this change (e.g. Mesh applications).

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>