Bluetooth: Validate hci conn validity before refering wakelock.

Found in a rare case there is possibility that remote device
sent disconnect on a connection and DUT is trying send data
on the same. In that case accessing some released wakelock
is causing issue. The current changes are to use locking
mechanism to validate the connection before acting on the
wake lock.

CRs-Fixed: 394651
Change-Id: I6a4188a7d0d05a8cfbe66d3680473d549157917a
Signed-off-by: Srinivas Krovvidi <skrovvid@codeaurora.org>