Bluetooth: bcm203x: Fix race condition on disconnect

Bluetooth: bcm203x: Fix race condition on disconnect

When disconnecting a bcm203x device we kill and destroy the usb-urb, however,
there might still be a pending work-structure which resubmits the now invalid
urb. To avoid this race condition, we simply set a shutdown-flag and
synchronously kill the worker first.

This also adds a comment to all schedule_work()s, as it is really not clear
that they are used as replacement for short timers (which can be seen in the git
history).

Signed-off-by: David Herrmann <[email protected]>
Acked-by: Marcel Holtmann <[email protected]>
Signed-off-by: Gustavo F. Padovan <[email protected]>