Bluetooth: Avoid race condition between hci registration/un-registration

Syncronized hci device registration/un-registration to avoid race
condition when called from mutiple paths/scenarios concurrently.

CRs-fixed: 375287
Change-Id: I920e7ade5b006b630ad8b02f953883a69b9677c7
Signed-off-by: Bhasker Neti <bneti@codeaurora.org>