Bluetooth: Block SCO disconnect operation on BT_CLOSED state.

Have maximum blocking time for shutdown operation as DISCONNECT_TIMEOUT.
During this period when SCO connection closes, the userspace will be
updated on the close operation. Existing approach of immidiate return
on shutdown call can cause synchronization issues on SCO state between
kernel and userspace. This happens when disconnect operation takes
more time at kernel space.

Change-Id: Id9e6a61c2c3d4ba2cf6da574fc49bc6894a8f96a
Signed-off-by: Srinivas Krovvidi <skrovvid@codeaurora.org>