Bluetooth : Ignore incomming events when the hci device is down

In error conditions Adapter state machine via Bluez tries to reset
the hci device, during that in some rare scenario if some pending event
comes from riva, the command complete apis tries to access some already
freed memory in reset sequence.

CRs-fixed: 369658
Change-Id: I5e9ce0a4322d07a26602c7f74b1484720f6b4d75
Signed-off-by: Bhasker Neti <bneti@codeaurora.org>