drivers: bluetooth: silabs: Potentially discard unimportant HCI events

Certain HCI events, like advertising reports, are of less importance than
others. This takes the discardable buffer pool into use for such events.
When the system is flooded with advertising reports, discarding some of
them ensures that the system can still handle other events.

Signed-off-by: Kalle Kietäväinen <kalle.kietavainen@silabs.com>