Bluetooth: drivers/h4: Add support for discarding certain events

If we are low on RX buffers and receive an LE Advertising Report or a
BR/EDR Inquiry response we should just discard this. If we don't
discard the event we increase the risk of deadlock where the RX
interrupt is enabled but bt_recv() is doing a synchronous HCI command
sending, i.e. waiting for a cmd_status/cmd_complete (which will never
come since the RX interrupt is disabled).

Change-Id: I6266625c9790d68bcf8e8718c8c36f127946c4c6
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>