staging: ozwpan: protect oz_pd_destroy

staging: ozwpan: protect oz_pd_destroy

oz_pd_destroy() is called mulitple times when network
down notifier gets triggered. Destroy workqueue is scheduled
from tasklet and also from oz_binding_remov function.
Protecting it by strictly scheduling workqueue only once.

Bug 1522180
Bug 1522708

Change-Id: I47c92a5e7ef1067d5dc4cdf67653a785eff34bca
Signed-off-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-on: http://git-master/r/426904
Reviewed-by: Bibhay Ranjan <bibhayr@nvidia.com>
Tested-by: Bibhay Ranjan <bibhayr@nvidia.com>
Reviewed-by: Mitch Luban <mluban@nvidia.com>
GVS: Gerrit_Virtual_Submit