Bluetooth: change cancel_delayed_work to cancel_delayed_work_sync

When bluetooth transport driver module is unloaded, it is possible
that workqueue is destroyed while callback for delayed work is
executing. Wait for that callback to complete before destroying that
workqueue.

Do not cancel work in the work callback function, else it will result
in deadlock.

Change-Id: I3471f6968315fef9ad0c5ce288b61db057ff8180
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>