BigSwitch: Semaphore on port status update

Adds a semaphore to the _set_port_status
function used by the Big Switch ML2 driver and
plugin. Without it, the async threads are
susceptible to deadlocking and causing the
known eventlet/mysql timeout exception.

Closes-Bug: #1300607
Change-Id: Ie89213a0bef37f9e7a30945df39c0f1b87a33b82