datapath-windows: Initialize reference count when enabling extension

datapath-windows: Initialize reference count when enabling extension

When the extension is initialized the global reference count, used for
preventing early deallocation of the switch extension, is set to 1.
Enabling and then disabling the extension leaves the latter reference
count to zero. Because of this the switch context fails to be acquired,
i.e OvsAcquireSwitchContext returns zero, and that affects driver's
communication to the userspace.

The solution is to initialize the reference count each time the extension
is enabled.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Nithin Raju <nithin@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/86
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>